Gittin 58 ~ Maternal Imprinting

From today’s rather sad page of Talmud comes this weird historical claim:

גיטין נח, א

מֵעִיקָּרָא, חֲשִׁיבִי דְרוֹמָאֵי הֲווֹ נָקְטִי בִּלְיוֹנָא דְגוּשְׁפַּנְקָא, וּמְשַׁמְּשִׁי עַרְסַיְיהוּ; מִכָּאן וְאֵילָךְ, מַיְיתוּ בְּנֵי יִשְׂרָאֵל וְאָסְרִי בְּכַרְעֵי דְּפוּרַיְיהוּ, וּמְשַׁמְּשִׁי

Initially the noblemen of Rome would keep an image imprinted on a seal by their beds and engage in sexual intercourse opposite that image, so that they would beget children of similar beauty. From this point forward, from the time of the Great Revolt, they would bring Jewish children, tie them to the foot of their beds, and engage in sexual intercourse across from them, because they were so handsome.

According to Rashi, these rings had images of beautiful people on them, and the hope was that the mother would become pregnant while looking at them:

בליוני דגושפנקי - צורות נאות שבחותמיהן כדי שתתעברנה כנגדן

All this is rather, strange, but not to a person who believed in maternal imprinting. Neither you nor I might not believe in such things, but pretty much everyone in the ancient and pre-modern world did. So today on Talmudology let’s take a deep look at the topic. And we begin with a red cow.

From here.

How to make a Red Heifer

The Parah Adumah, the red heifer, was used in several ceremonies in the Temple. It was, however, a rare animal. In today's page of Talmud there is a detailed discussion as to whether a red heifer born to an idol-worshipper could be used. The concern was that the heifer, or one of its ancestors, might have been used by the idol-worshipper for beastiality. Should this have happened, it was forbidden to use the red heifer as a sacrifice. The Talmud relates that in fact an idol-worshipper called Daba ben Natina had sold a red heifer to his Jewish neighbors. To insure that the heifer's mother had not been the object of beastiality, the pregnant cow had been watched "משעה שנוצרה" – from the moment it was impregnated. Then comes an obvious question: how could anyone be sure that the cow was indeed pregnant with a red calf which would warrant safeguarding her? Perhaps the calf  would be born another color? 

Here is the answer: "כוס אדום מעבירין לפניה בשעה שעולה עליה זכר" -"while the mother was copulating, the farmer would show her a red cup." That would insure that she would give birth to a little red calf that would grow into a bigger red heifer. The belief that what a mother sees during conception and gestation will affect her offspring is called maternal imprinting or psychic maternal impressions, or mental influence, or maternal imagination, or (my favorite) maternal fancy. 

“The history of the belief in maternal impressions is one of great antiquity...it is also one of practically world-wide distribution. [It can be found in] such far apart lands as India, China, South America, Western Asia and East Africa..the Esquimaux, the Loango negros, and the old Japanese.”

— John William Ballantyne. Teratogenesis: an Inquiry into the Causes of Monstrosities. Edinburgh, Oliver & Boyd 1897. 24

Maternal Imprinting in jewish sources

The earliest mention of maternal imprinting is the story of Yaakov and his division of the goats he watched for his uncle Lavan (Gen 30:25-43, 31:1-12).  As wages, Yaakov asked for all the speckled goats, while Lavan would get to keep the plain ones.  Yaakov then took several wooden rods from which he peeled the bark, and left these now speckled rods in front of a water trough.   The female goats stared at the rods while they are drinking and mating, and this in turn caused them to give birth to speckled kids, all of which Yaakov got to keep. That's how maternal imprinting works.  

The Midrash and Talmud are replete with the belief in maternal imprinting. Perhaps the most famous story is that of Rabbi Yochanan (~180-279 CE) who would regularly sit in front of the mikveh (ritual bath). He did this so that women leaving there would see him, and be blessed with sons as handsome as he was.

בבא מציעא פד, א

ר' יוחנן הוה אזיל ויתיב אשערי טבילה אמר כי סלקן בנות ישראל מטבילת מצוה לפגעו בי כי היכי דלהוו להו בני שפירי כוותי גמירי אורייתא כוותי

Rabbi Yochanan would go and sit by the entrance to the ritual bath. He said to himself: When Jewish women come up from their immersion [after their menstruation,] they should see me first so that they have beautiful children like me, and sons learned in Torah like me. 

Rabbi Akiva used maternal imprinting to save a king from a rather embarrassing situation:

מדרש תנחומא נשה, ז

מַעֲשֶׂה בְּמֶלֶךְ הָעַרְבִים שֶׁשָּׁאַל אֶת רַבִּי עֲקִיבָא, אֲנִי כּוּשִׁי וְאִשְׁתִּי כּוּשִׁית וְיָלְדָה לִי בֵּן לָבָן, הוֹרְגָהּ אֲנִי, שֶׁזָּנְתָה תַּחְתַּי. אָמַר לוֹ: צוּרוֹת בֵּיתְךָ שְׁחֹרוֹת אוֹ לְבָנוֹת. אָמַר לוֹ: לְבָנוֹת. אָמַר לוֹ: כְּשֶׁהָיִיתָ עוֹסֵק עִמָּהּ, עֵינֶיהָ נָתְנָה בְּצוּרוֹת לְבָנוֹת וְיָלְדָה כַּיּוֹצֵא בָּהֶם. וְאִם תָּמֵהַּ אַתָּה בַּדָּבָר, לְמַד מִן צֹאנוֹ שֶׁל יַעֲקֹב, שֶׁמִּן הַמַּקְלוֹת הָיוּ מִתְיַחֲמוֹת, שֶׁנֶּאֱמַר: וְיִחֲמוּ הַצֹּאן אֶל הַמַּקְלוֹת (בראשית ל, לט). וְהוֹדָה מֶלֶךְ הָעַרְבִים וְשִׁבַּח לְרַבִּי עֲקִיבָא

A king of Arabia once asked Rabbi Akiva, “I am black and my wife is black, yet she gave birth to a white child. Shall I have her executed for infidelity?” Rabbi Akiva responded by inquiring if the statues in his house where white or black. He said to Rabbi Akiva that they were white.  Rabbi Akiva explained to the king that during conception his wife's eyes were fixed on the white statues and so she bore a white child...the king agreed and praised Rabbi Akiva

Maternal Imprinting in Greek Thought, and Beyond

In 1998 Professors Wendy Doniger and Gregory Spinner published perhaps the most comprehensive review of imprinting, in a paper titled Misconceptions: Female Imaginations and Male Fantasies in Parental Imprinting. They noted Empedocles, who lived in the fifth century BCE, wrote that 

the fetuses are shaped by the imagination of the women around the time of conception. For often women have fallen in love with statues of men, and with images, and have produced offspring which resemble them.

Soranus of Ephesus, another Greek physician who lived in Rome and Alexandria (and who was a contemporary of Rabbi Yochanan) firmly believed in imprinting, both for animal husbandry and in humans:

Some women, seeing monkeys during intercourse, have borne children resembling monkeys. The tyrant of the Cyprians, who was misshapen, compelled his wife to look at beautiful statues during intercourse and became the father of well-shaped children; and horse-breeders, during covering, place noble horses in front of the mares.

Let's jump forward a millennium.   In 1282 it was reported that an infant was born with hair and claws like a bear. The Pope at the time "straightway ordered the destruction of all pictures of bears in Rome." This story is from John Ballantyne, a Scottish physician who in 1897 published Teratogenesis: an Inquiry into the Causes of Monstrosities. According to Ballantyne, in the seventeenth century, the belief in maternal impressions "reigned supreme."  Here is another example of the kind of thing it led to:

And then things get even weirder:

In 1726 the matter of maternal impressions was brought still more prominently before the profession and the public in England in connexion [sic] with the notorious case of an "Extraordinary Delivery of Rabbets" which was alleged to have occurred in the case of Maria Tofts of Godlyman in Surrey; she had a great longing for 'Rabbets"in early pregnancy.

Pregnancy and the fetus

Maternal imprinting is a rather extreme form of what we all know to be true; that what happens to a pregnant mother affects the fetus she is carrying. Here are two of the countless examples of this.  If a mother drinks enough alcohol while pregnant, the fetus will be born with Fetal Alcohol Syndrome. This syndrome includes facial abnormalities, growth delays, abnormal development of organs, and reduced immunity.  If a mother is infected with measles (rubella) early in pregnancy, the baby will likely be born with congenital rubella syndrome, which includes cataracts, congenital heart disease and brain damage.  But these examples do not imply that a woman who eats eggs will have children with large eyes or that a woman who eats an esrog will have fragrant children, as the Talmud (Ketuvot 60b) declares. Some things a pregnant mother does will have a huge effect on the fetus she carries. Some won't have any affect at all.  

John Ballantyne, the Scottish physician concluded his book noting, as we have, that of course there are some things a mother does that will affect fetal outcome. "To this extent" he wrote, "I believe in the old doctrine of maternal impressions; this is, I think, one grain of truth in an immense mass of fiction and accidental coincidence."

It's not the red cup after all

Let’s return to the red heifer. It would seem that all you needed to produce a Parah Adumah was to place a red cup next to the mating cattle.  So why didn't every farmer use the red cup protocol to breed a red heifer? After all, these animals commanded fantastic prices because of their rarity. The answer offered by the Talmud is that the red cup protocol only worked with a herd of cattle that were known (במוחזקת) to produce red heifers.  Without that breeding history, the red cup protocol was useless. So this really wasn't about the red cup. It was about the genes, and that's the kind of parental imprinting that really does work.

“A surprising large number of people, in different cultures and over many centuries, have believed that a woman who imagines or sees someone other than her sexual partner at the moment of conception may imprint that image upon her child- thus predetermining its appearance, character or both.”

— Doniger W. Spinner G. Female Imaginations and Male Fantasies in Parental Imprinting. Daedalus 127 (1), No. 1, Science in Culture (Winter, 1998). 97-129

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[Special thanks to Rabbi Dr. Eddie Reichman, medical historian and Talmudology reader who has been researching maternal imprinting for years, and was kind enough to share his material.]

Sotah 31a ~ What Can a Fetus See?

In tomorrow’s post we will study an aggadic statement, that is to say, a homiletic teaching, that is not to be taken literally - or so one would think.  In that daf the Talmud discusses the miracles which occurred as the Children of Israel crossed through the parted waters of the Red Sea. Rabbi Meir taught that even a fetus in its mother's womb praised God, saying "This is my God and I will glorify him." Now we might have considered this a homiletic teaching that is meant to simply express a degree of amazement and thanks.  But the Talmud then asks a question that suggests Rabbi Meir meant what he said more literally:

סוטה לא, א

והא לא חזו! אמר רבי תנחום כרס נעשה להן כאספקלריא המאירה וראו

These fetuses in the womb could not see the Divine presence, so how could they sing praise? Rabbi Tanchum said: Their mother's abdomen became as clear as glass for them and they were able to see.

While Rabbi Tanchum suggested that it takes physical sight rather than emotional insight to see the divine, it turns out that the fetus can see - and hear, while still in the womb.

Increased Fetal Heart Rate in Response to Light

In 1980, two Israelis published a preliminary report on the response to light of ten fetuses between 38 and 43 weeks' gestation. They inserted an amnioscope through the cervix and shone a light into the womb for thirty seconds while monitoring the heart rate of the little fetus. They found that eight of the ten little fetuses had an acceleration of their heartbeat in response to the light. That's interesting you say, but hardly what Rabbi Tanchum was describing. And you'd be correct.  So let's turn to some other studies.

Increased Fetal Brain Activity in Response to Light

A review in the European Journal of Obstetrics and Gynecology and Reproductive Biology published in 1996 was sceptical that the fetus could see much of anything while inside the womb: 

In utero visual stimulation appears to be very limited...in a dark room the amniotic cavity may be candled with a torch light, especially in the case of a polyhydramnios [an excess of amniotic fluid]. Measurements performed during rat and guinea-pig gestation have demonstrated that if only 2% of incoming light was transmitted in utero below 550 nm, this value increases with wavelength of the signal to reach 10% around 650 nm. Thus, a limited portion of external light may reach the human fetal retina when eyelids are open (this behavior starts at 20 weeks) or through the eyelids. 

But in 2003 a group of researchers from the United Kingdom (with apparently nothing else to do for amusement) built a light source from a "cardboard tube lined with non-conducting aluminised plastic, resulting in a light intensity of 1,100–1,200 Lux at the maternal abdomen as measured with a hand-held light meter." After an ultra-sound confirmed that the fetus was looking forward (really, they did this too) they turned the light on and off. And all this took place while the mother and her in-utero child were lying in a functional MRI scanner, which was used to look for activation of the little fetal brain in response to the light. Of the nine subjects they tortured in this way, one could not be analyzed due to motion, three did not show any significant activation, and five showed significant activation. Oddly, none of the fetal brains that responded showed any activation of the occipital lobe, that part of the brain in which the primary visual cortex is located and which responds to light.  Instead, it was the fetal frontal cortex showed a response to the light being shone.  Hmmm.

The Fetal Response to Sound

So much for vision. Researchers have also studied what - if anything - a fetus may be able to hear.  A group from Rambam Hospital and the Technion in Haifa studied the effect of music on fetal activity. Back in 1982 they took twenty pregnant women and played them either 25 minutes of nothing, or 25 minutes of classical or pop music through headphones. If you are wondering, the music was either a canon and songs composed by Pachelbel, or "the pop-hits of the [sic] Boney-M." (Give yourself an extra point if you can recall any of the pop hits of the Boney M.) Anyway, they played the music in random sequence and monitored the fetus for breathing and body movements.  They found that compared to no music, when music was piped into the mothers' ears there was a significant increase in the breathing movements of the fetus, but there was no difference between classical and pop music.  

“..it seems that the fetus moves into a more active state when music is played to the mother.”

— Zimmer, EZ. et al. Maternal Exposure to music and fetal activity. Europ. J. Obstet. Gyec. Reprod. Biol. 1982 (13) 210.

And remember the experiments with cardboard tubes shining light into the womb of forward facing fetuses? Well that same group also performed functional MRI scanning of the brains of a group of fetuses but this time they strapped "an MRI compatible headphone" to the maternal abdomen (or the maternal ears, as a control) and played 15 seconds of music. (The paper does not specify the kind of music that was chosen. I do hope it wasn't the Boney-M.) Five of the twelve fetuses that had music piped into their mother's abdomen showed activation of the temporal lobes, but despite this low number the authors enthusiastically concluded that their study showed "...that brain activity can be detected in response to stimulation prenatally..." 

“A ray of hope flitters in the sky
A tiny star lights up way up high
All across the land dawns a brand new morn
This comes to pass when a child is born”

— Boney M. When a Child is Born, 1981.

Giving Thanks - Thanksgiving

The Talmud describes how the Crossing of the Red Sea was a miracle of such extraordinary nature that even in-utero fetuses joined in singing a prayer of thanks with the Children of Israel. In his famous introduction to the tenth chapter of Sanhedrin, Maimonides describes how aggadah should not be taken literally. Instead, a deeper message should be sought. And so, over Shabbat, perhaps you can discuss what you are thankful for. For what blessings in your life might a fetus open its eyes and see, or say thanks while still in its mother's  womb? Now that I think of it, that's a question that everyone should answer.

“Human fetuses are, to a certain extent, able to memorize certain sensory properties...Despite the fact that they have only very short periods of wakefulness and that their brain is not mature enough to integrate sensory experiences, several experiments suggest that this does not prevent pre- and perinatal learning.”

— Lecanuet, J, Schaal B. Fetal Sensory Competencies. European Jopurnal of Obstetrics and Gynecology and Reproductive Biology 1996. 68: 1-23

Ketuvot 60b ~ Everything is Bad For You

On this page of Talmud we read of a number of things that, if done by a pregnant mother, may injure (or rarely, improve) her unborn child.

תלמוד בבלי כתובות דף ס עמוד ב - סא עמוד א

דמשמשא בי ריחיא - הוו לה בני נכפי, דמשמשא על ארעא - הוו לה בני שמוטי, דדרכא על רמא דחמרא - הוו לה בני גירדני, דאכלה חרדלא - הוו לה בני זלזלני, דאכלה תחלי - הוו לה בני דולפני, דאכלה מניני - הוו לה בני מציצי עינא, דאכלה גרגושתא - הוו לה בני מכוערי, דשתיא שיכרא - הוו לה בני אוכמי, דאכלה בישרא ושתיא חמרא - הוו לה בני בריי, דאכלה ביעי - הוו לה בני עינני, דאכלה כוורי - הוו לה בני חינני, דאכלה כרפסא - הוו לה בני זיותני, דאכלה כוסברתא - הוו לה בני בישרני, דאכלה אתרוגא - הוו לה בני ריחני. ברתיה דשבור מלכא אכלה בה אמה אתרוגא, והוו מסקי לה לקמיה אבוה בריש ריח

A woman who copulates in a mill will have epileptic children. [A woman] who copulates on the ground will have children with long necks. [A woman] who treads on the excrement of a donkey will have children who lose their hair.  [A woman] who eats mustard will have children who are gluttons. [A woman] who eats cress will have children with teary eyes.  [A woman] who eats small fish will have children with fluttering eyes.  [A woman] who eats clay will have ugly children.  [A woman] who drinks beer will have dark children.  [A woman] who eats meat and drinks beer will have children who are healthy.   [A woman] who eats eggs will have children with large eyes.  [A woman] who eats fish will have charming children.   [A woman] who eats celery will have radiant children.  [A woman] who eats coriander will have fat children.  [A woman] who eats an esrog will have fragrant children...

Everything is Bad For You. Or Good For You.

In their highly entertaining 2013 paper, Schoenfeld and Ioannidis looked at 50 common ingredients from random recipes found in The Boston Cooking-School Cook Book.   Then they searched for any recent scientific studies that evaluated the relation of each ingredient to the risk of cancer. And what did they find?

Eighty percent of ingredients from these randomly selected recipes had been studied in relation to malignancy.  (Those ingredients that had not been studied tended to be more obscure, like hickory or terrapin.)

Thirty-nine percent of studies concluded that the studied ingredient conferred an increased risk of malignancy; 33% concluded that there was a decreased risk, 5% concluded that there was a borderline statistically significant effect, and 23% concluded that there was no evidence of a clearly increased or decreased risk. In most (80%) of the studies, the statistical effect was weak.  As you can see in the table, the same ingredient (like tomatoes, tea, carrots and coffee) was found in different studies to both increase and decrease the risk of cancer. 

Effect estimates by ingredient. From Schoenfeld and Ioannides. Is everything we eat associated with cancer? Am J. Clin. Nutrition 2013:97;127-34. 

The authors concluded that:

“Nutritional epidemiology is a valuable field that can identify potentially modifiable risk factors related to diet. However, the credibility of studies in this and other fields is subject to publication and other selective outcome and analysis reporting biases, whenever the pressure to publish fosters a climate in which “negative” results are undervalued and not reported. Ingredients viewed as “unhealthy” may be demonized, leading to subsequent biases in the design, execution and reporting of studies.”

So that's the lesson: Be really careful when ascribing risk or benefit to commonly found ingredients. And with that warning, let’s analyze the passage in today’s daf:

THE RISK OF EPILEPSY

According to the Talmud, copulating in a mill is a risk factor for epilepsy.  Although there has not been a significant attempt to categorize the causes of epilepsy, one recent paper suggested that the etiology of this condition be broken down into four types:

Idiopathic (predominantly genetic or inherited in a complex way)

Symptomatic (acquired or genetic, together with gross anatomical or clinical features)

Provoked (the predominant cause is environmental)

Cryptogenic (cause has not been identified)

The Talmud is describing the mill as one of the environmental causes of epilepsy. While no such link has ever been suggested outside of the Talmud, the claim that epilepsy has an environmental cause is certainly plausible.

OTHER PRE-CONCEPTION ENVIRONMENTAL ASSOCIATIONS WITH CHILDHOOD OUTCOMES

It is increasingly clear that some major diseases in later life – like diabetes, high blood pressure and heart disease- have a basis in the impaired growth of the fetus. For example, a long-term British study showed that those infants who had low birth weights had relatively high death rates from coronary heart disease in adult life. Low birth rate has also been associated with the later development of diabetes, high cholesterol, and high blood pressure. In today’s daf, the Talmud claims an association between some foods ingested by a woman and certain characteristics or health traits in the later development of children she may carry. Again, there is no evidence that the details here are correct, but there is no reason to exclude such an association ab initio.

PICA – clay makes you ugly

Pica is the medical term for the craving and ingestion of foods or substances that have no nutritious value. Pregnant women engage in pica behavior all over the world. One study found that three-quarters of pregnant women in Kenya ate soil on a regular basis, and that this practice had a strong relationship to fertility and reproduction. In a study of 128 pregnant women conducted in a rural America, about a third practiced pica and clay was sometimes eaten together with other substances –a practice called polypica. Although women reporting daily pica practice were significantly more likely to have lower prenatal hematocrits than women who did not practice pica, no specific pregnancy complication was associated with the practice of pica. Although the authors did not report on whether the children were more likely to be ugly, the evidence suggests this is not related to pica, and so this claim of Talmud is unlikely to be true.

Pica Frequency. From Corbett et al. Pica in Pregnancy. Does it affect pregnancy outcomes? American Journal of Maternal Child Nursing. 2003: 28 (3); 183-189.

Fetal Alcohol Syndrome

The relationship between the ingestion of a pregnant woman’s ingestion of alcohol and the growth of the fetus is clear. It is a terrible idea to drink when you are pregnant. If you drink while pregnant, the fetus will be born with Fetal Alcohol Syndrome, which includes facial abnormalities, growth delays, abnormal development of organs, and reduced immunity.  The Talmud suggests that drinking beer before (or during?) pregnancy leads to “dark children” while drinking beer and eating meat will lead to healthy children. Do not follow this advice.  Drinking beer while pregnant is a  really bad idea, at pretty much any dose.

“Despite the many gains in knowledge, we still do not know if there is a “safe” dose of alcohol that can be consumed by pregnant women without risking damage to their unborn children. Until such a safe dose, if it exists, can be determined, the only responsible advice to women who wish to become pregnant and to those who are pregnant is to avoid alcohol use entirely. ”

— Enorch Gordis MD, Then Director of The National Institute on Alcohol Abuse and Alcoholism. Alcohol Alert #50, 2000

Eggs and Etrogs

The final intriguing association that is made is between eggs and large eyed children, and etrogs and fragrant children. Here the assumed mechanism is clear. You are what you eat – literally. Eggs have big yellow yolks, so if a woman eats them, her children’s eyes will mimic the egg – and appear to have large pupils surrounded by the white of the conjunctiva. Similarly, by eating a fragrant etrog, the body literally becomes fragrant. It’s a lovely theory really, but totally without of any basis in fact. “Like causes like” is as unlikely as “that which causes the same symptoms leads to a cure.” The latter is called homeopathy, and there is no scientific basis to it whatsoever. (Before you send me that angry email, read the sentence again. I did not say that it is not effective. It is indeed effective; as effective as placebo – and no more. But there is no scientific evidence that homeopathy is any better than sham treatment, that is, a placebo). The latest evidence to show that there is no benefit of homeopathy comes from a lengthy report (actually a series of reports) from the Australian National Health and Medical Research Council. We’ve wasted enough money on chasing the scientific study of homeopathy; let’s not waste still more seeing if eggs make your kids eyes larger because “like causes like”…

The Talmud's list of possible environmental causes is a joy to read, but is it true? Without a doubt yes. And certainly no.

Yevamot 98b ~ On the Formation of Twins

Buried in a discussion of paternity, Rava states that the mechanism of the development of twin brothers is as follows:  "דטפה אחת היה ונחלקה לשתים"  - "there was one drop of semen that split into two".

Aristotle (384-322 BC) believed that twins were caused by too much seed (which does make a certain intuitive sense I suppose). In the thirteenth century the German theologian Albertus Magnus updated this theory, and suggested that twins were generated when the woman experienced too much sexual pleasure during intercourse. This redistributed the seed throughout the uterus, resulting in twins. 

We now know that there are actually only two ways that twins are formed, and neither depends on sperm splitting, too much seed, or an abundance of sexual pleasure. In monozygotic or identical twins (though they are not always literally identical), one sperm fertilizes one egg, and the resulting zygote then divides into two. Since the two offspring zygotes originated from the same "mother" zygote, they are genetically identical - and hence the twins (or triplets, if the zygote splits three ways) are identical and of course of the same sex.

Formation of identical (monozygotic) twins. Courtesy Womenshealth.gov

In dizygotic or fraternal twins, two sperm fertilize two eggs and the two resulting zygotes grow together in the same womb at the same time. They are not genetically identical, and are really two siblings who just happen to be sharing a womb at the same time. 

Formation of fraternal (dizygotic) twins. Courtesy Womenshealth.gov

That's it. No sperm splits.  Sorry.

Fun facts about twins, Israeli and others

• For reasons that are not clear, the rate of dizygotic twins varies across the world, (highest in sub-Saharan Africa, lowest in Asia) and varies across women by body habitus (taller women are more likely to have twins - who knew?) and age (older women are four times more likely to have twins) and varies across time (possibly highest when conception is in the summer and fall).

Twinning Rates in the USA, Europe, Australia and Asia. From Hoekstra et al. Dizygotic Twinning. Human Reproduction Update 2007.14; 37-47.

• From 1915, when records were first started, through the 1970s, the rate of twin births in the US was a constant 2% of all births. But in the early 1980s the number of twin births in the US more than doubled, and today that rate is over 3%. The cause, as you may have guessed, is the increased use of reproductive technologies.

“One in every 30 infants born in 2009 was a twin”

— US Dept of Health and Human Services. NCHS Data Brief Jan 2012

• The states that saw the greatest increase in twin birth rates were Connecticut, Hawaii, Massachusetts, New Jersey and Rhode Island. In these states, the twin birth rate more than doubled!

Percentage Increase in Twin Birth Rates 1980-2009. From NCHS Data Brief #80. Jan 2012.

• Finally, although the rate of twin births has increased for women of all ages over the last three decades, in women older than 40 the rate of twin births increased by 200%. There are at least two causes for this dramatic rise. First, as more women postpone childbirth, more are having children later in life, and older women, as we have already seen, are more likely to have (dizygotic) twins. And secondly, more older women are using assisted reproduction techniques, which also increases the rate of twin conceptions.

• In Israel, the incidence of twin births increased three fold in the three decades before 2001, a rate even higher than the increase in the US. The authors of a 2004 study on multiple births in Israel concluded that the "increased incidence of multiple births is explained by the special significance attributed to motherhood in Israeli society, which is met by the socio-political milieu and the availability of assisted reproductive technologies". Indeed.

• The incidence of twin births in higher in Jewish than it is in Arab Israelis.  This is likely explained by their greater use of assisted reproductive technologies (like in-vitro fertilization).  

Incidence of twin births in Jewish and Arab populations of Israel, 1994-2001. From Blickstein I, Baor L. Trends in multiple births in Israel. Harefuah 2004; 143 (11): 794-798.

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The Talmud wasn't exactly accurate about how twins are formed, but elsewhere it does capture the sense of wonder that so many of us feel as we hold a newborn baby (or two). It's a transcendent moment in which we may, for a fleeting moment, feel the divine.   

“תנו רבנן שלשה שותפין יש באדם הקב”ה ואביו ואמו. אביו מזריע הלובן...אמו מזרעת אודם ... והקב”ה נותן בו רוח ונשמה וקלסתר פנים...

Our Rabbis taught: There are three partners in the creation of a person, the Holy One, blessed be He, the father and the mother. The father supplies the white...the mother supplies the red...and the Holy One, blessed be He, supplies the spirit and the breath and the beauty...”

— Niddah, 31a