Now that I’m nursing a newborn again, I’ve renewed my fascination with the science of breast milk. In particular, “lactocrine programming;” the idea that hormonal signals in Mama’s milk can “program” Baby’s body and behavior. So many new developments in the last few years since UJ was born!
Today’s question: Does breastfeeding make kids more fertile later in life?
Theories abound with evidence accumulating from various studies on humans and other mammals. Researchers Katie Hinde and Danielle LeMay, at Harvard and UC David respectively, offer some intriguing insights in their SPLASH! Milk Genomics blog.
The upshot: The first few days after birth are a crucial window for the development of reproductive tissues that will become part of the cervix, uterus, endometrium, and other parts. These tissues start growing when they receive signals from a hormone called relaxin (a multipurpose hormone, it also relaxes blood vessels and ligaments during pregnancy, opens the cervix, and much more).
There’s relaxin in the milk! report Hinde and LeMay, referring to a study on pig milk:
Here’s the crazy part: relaxin is delivered by the mother via her milk. Piglets that are allowed to suckle have relaxin in their blood stream, but not piglets fed a milk-replacer . Relaxin activity in pig milk is highest in the first few days of lactation, and is similar to findings from dogs and humans. Experimental manipulations have shown that as little as one colostrum feeding in the first 12 hours after birth can make a difference. For example, just a single colostrum feeding bout in the first hours after birth allows for typical cervical cell proliferation and development—an important predictor of future litter size.
But does access to milk really predict future fertility? It did in a recent pig study involving over 1,500 litters, say Hinde and LeMay:
Female pigs with limited access to maternal-origin hormones via milk as piglets had reduced litter size as adults. So, it is safe to conclude, at least in pigs, that the number of babies born in any generation was partly programmed by their grandmothers via milk hormones.
It’s mind-boggling, the possibility that access to breast milk in infancy — or at least colostrum (the thick milky fluid produced in the first days after birth, which also contains relaxin) may have an impact on our kids’ reproductive development.
Not that these preliminary (porcine) studies say anything conclusive about humans. For that, we’d need to address many more questions: Do women whose mothers never attempted breastfeeding have more fertility problems — or fewer kids — than their breastfed peers? Do they have a different growth trajectory in puberty? How does relaxin affect reproductive development in sons? Can breastfeed girls bear children later in life than those who weren’t breastfed? How long does a kid need to nurse to receive reproductive benefits — the first day after birth, a few days, the first week? More?
Further research is warranted. Even if turns out that fertility is only slightly enhanced among breastfed kids (that’s my bet, anyway), it’s more fuel for the breastfeeding movement.
Check out my video and excerpt from Chocolate Lovers on Big Think.
Check out in BOING BOING one of my favorite excerpts from Chocolate Lovers — on the very weird science of what fetal cells do in mothers’ bodies.
The best holiday gift: DO CHOCOLATE LOVERS HAVE SWEETER BABIES gets a STARRED REVIEW in Library Journal! Many thanks to Library Jrnl and the reviewer, Julianne Smith.
What a charm! Science writer Pincott (Do Gentlemen Really Prefer Blondes?) tackles some myths and legends associated with pregnancy and compares them to peer-reviewed research on the matter. The book covers such questions as: “Do men prefer babies who resemble them?” “What does a baby’s birth season predict?” and “Do bossy broads have more sons?” This is an enjoyable, insightful, and fascinating look at pregnancy that explains what we know and identifies what we don’t. In discussing topics from stretch marks to mama’s boys, Pincott takes a conversational tone, making the science readily available to all readers. An ideal acquisition for public libraries, a great gift for expectant parents, and the perfect choice for the doctor’s waiting room, this winning title deserves some talking up. Way more fun than What To Expect.
Does stress sharpen your baby’s mind—or dull it? Can you predict your baby’s temperament? Why are babies born in the darker months of the year more likely to be risk-takers? Are bossy, dominant women more likely to have boys, which skinny women have more girls?
When our baby was six months old, my husband and I couldn’t walk down the street without stopping to accept a compliment. Baby was something of a local celebrity or a minor deity. Free pizza and donuts, the choicest table in the restaurant, a bracelet, stickers, sunglasses, a spot at the front of the line — these were some of the offerings made by the infant’s admirers. Ten months have now passed, and now she’s a toddler. To me, she’s as cute as ever, and she still receives a lot of kisses and waves from strangers. But, we’ve got to face it — she doesn’t attract the same the solicitous stop-in-the-street simpering that she did in her first year. How much longer will it be before everyone stops thinking she’s so adorable?
This question happens to be the subject of a new study on children’s faces. A trio of psychologists— Lu Zhu Luo, Hong Li, and Kang Lee — in China, and at the University of Toronto, recruited 60 men and women and showed then a large sample of children’s faces ranging from infants to 6-and-and-half-year-olds. The participants were asked to rate each face’s likeability (How much do you like the face?) and attractiveness (How attractive is the face?). The researchers wanted to know if younger kids would be given higher attractiveness ratings than older kids — and, if so, at what age does the cut-off happen from over-the-top adorable to merely cute?
The answer? Yes, it’s as expected. Men and women rated infants as cuter than toddlers, who, in turn are rated as cuter than young children. The big drop-off in cuteness appears to happen somewhere between preschool and kindergarten. The researchers identified it as approximately age 4 ½.
What happens to school-age kids?
Their facial structure changes, according to the Luo and his colleagues. Infants have a special set of features, such as a protruding forehead, a large head, a round face, big eyes, and a small nose or mouth. As a species, we have evolved to be attracted to babyish features and find them adorable. These cues make us feel soft and protective, whether or not we’re biologically related — which increases the likelihood of the baby’s survival. (Indeed, studies have found that infants that have tiny eyes, flat foreheads, and square faces, for instance, are less likely to receive attention.) Pleasingly, our tendency to prefer infantile faces even extends to infant faces of other races.
Facial cranial growth is gradual, as is a child’s independence from constant care-giving. Children’s faces lose some of their universal appeal right around the age that they don’t need it anymore to (merely) survive — somewhere around kindergarten-age. Incidentally, this interval — four to five years — is the same as natural birth spacing — when our foremothers would become pregnant with their next baby.
Today I was thrilled to see a mention of Chocolate Lovers in one of my favorite parenting sites, Urban Baby.
For months I’ve had a hunch that having a baby has been better for my husband’s image than mine. I don’t mean his looks. Neither of us has had much time to sleep and shower and pay attention to clothing and hair. What I mean is that I think first impressions favor fathers more than mothers. Pushing our baby along in the stroller, holding a coffee cup and singing, I fall into an easy stereotype, but my husband doesn’t. Yes, in terms of public image, I believe a man benefits more from being a father than a woman benefits from being a mother.
Am I right?
A couple years ago, Ariane Kemkes, a researcher at the Tholius Institute for Research in Applied Demography in Scottsdale, Arizona, asked the same question. Kemkes wondered what would happen if you took a picture of a parent sitting next to his or child, and impartial judges rate that parent in terms of attractiveness, smarts, success, and so on. Then, she wondered, what would happen if you crop the child out of the photo and ask a different set of judges to rate them on the same criteria?
Are men and women judged differently when they’re with kids than without? And if so, do fathers benefit from a bigger boost in their social image than do mothers?
The results are intriguing to us new parents.
Men perceive mothers and fathers differently than women do. Looking at a photo of mother and child, male judges are 2.4 times more likely than female judges to believe that the woman is committed to family. Female judges were more cynical and critical of other women’s maternal commitment (but more interested in meeting them). Surprisingly – and to my relief – both sexes are marginally (1.1 times for men, 1.2 for women) more likely to think a woman looked more attractive with a child than when she was alone. Men, however, were more ambivalent abot meeting women if they were mothers. A mother was also perceived by judges of both genders as slightly, but not significantly, more faithful, honest, and mature.
But what about her mind? Here comes the crux of my argument about the drawbacks of motherhood. If a woman was paired with her child, both male and female judges perceived that woman to be less ambitious than if she was alone. The presence of a child around a woman reduced the woman’s likelihood of being regarded as ambitious by as much as 30 percent. The assumption by men is unsurprising, but that the stereotype is held by other women is startling. The results may make one pause on bring-your-child-to-work day.
And now, what about men — what does fatherhood do to their image?
Only good things, as I presumed.
Men with children were perceived by all as being committed to family. Interestingly, fatherhood was good for a man’s social life. Men were 1.2 times more interested in meeting fellow a man with a child than the same man without a child. And here’s another perk of fatherhood: A man with a child is perceived to be of a higher social status. This comes from judges of both genders. Fathers are also believed to be more faithful, mature, honest. They’re also thought to be more generous – a perception not transferred to women with children.
Kemkes sums up the stereotype: females most often associate maternity as conflicting with career and leisured activities, while males emphasize financial sacrifices. A childless woman is perceived as ambitious and a childless male is perceived as cash-strapped, immature, or having a lower social status. For men, there is a strong association, explainable in evolutionary terms, between reproductive and financial status that does not exist for women.
Moreover, as Kemkes points out, men who are fathers are perceived as more generous than their childless counterparts because emotionally unstable men are more possessive and monopolize resources – traits not associated with fatherhood. Being a dad makes a man appear more “prosocial”; that is, generous and willing to cooperate. Men are likelier to want to meet dads than childless men because fatherhood lowers expectations of inter-male competition. It’s now established that fatherhood is linked to lower testosterone levels, especially in the first months after a baby’s birth.
There’s a lesson in research of this kind. While parenthood generally boosts the social image of both genders, it still cripples career women more than men. Although more fathers are taking care of the their kids while the mother goes to work, the stereotype that mothers aren’t as ambitious still hasn’t budged – in part, perhaps, because many workplaces continue to make it difficult to excel in both. So the disappointing fact remains: A man who prominently features a photo of himself with his children on his desk at work is doing more for his career than a woman who does the same.
*If you like this blog, click here for previous posts. If you wish, check out my new book — hot off the press! — Do Chocolate Lovers Have Sweeter Babies?: The Surprising Science of Pregnancy.
Early one evening late in my second trimester of pregnancy, I was standing in the dairy aisle of the grocery store, with one hand on my back and the other over the kicking baby in my distended belly. A young man approached me, initiated a conversation about the World Cup, and, casually, asked me if I’d like watch the game with him that weekend. “You’re pretty!” he whispered. I was shocked.
I wasn’t putting out a sexy vibe. (Not at all.) I had assumed that any male attention I receive in late pregnancy, including that from my husband, would be friendly, not sexual. Why would a man who is not the expectant father think pregnancy is sexy? But then other women told me similar stories about how they got hit on in third trimester. So I decided to look into it, and it turns out that a study on sexual attraction to pregnancy has recently come out.
A team of Swedish and Italian doctors, led by Emmanuele Jannini and Magnus Enquist, recruited nearly 2,200 men who had joined online fetish groups such as alt.sex.fetish and alt.sex.fetish.breastmilk. They presented a questionnaire that asked the respondents questions about their preferences for pregnant and lactating women. The survey also asked for the sex and age of each sibling, and whether the sibling is a full sibling or not (half-sibling or adopted child). Most respondents reported both a pregnancy and a lactation preference. The average age at which respondents became aware of their preference was about 18 years.
What Jannini and Enquist and their colleagues were searching for was evidence that there was something special about the upbringing of men that are secually aroused by pregnancy. They knew that a specific stimulus early in life can elicit sexual behavior when that animal reaches sexual maturity. For instance, goats that are raised by sheep are sexually aroused by sheep only. This is called sexual imprinting.
Is it possible that boys that are raised by women who are pregnant for much of their childhoods are unusually attracted to pregnant women?
It turns out, what’s good for the goat is good for the guy. The more exposed a man was to his mother being pregnant and breastfeeding when he was between 1.5 and 5 years old, the more likely he is, as an adult, to be sexually attracted to pregnant and breastfeeding women.
A younger sibling is the key to early exposure. The respondents who eroticized pregnancy and breastfeeding had significantly more younger siblings than expected by chance. Respondents with one sibling were older than their sister or brother in 66 percent of cases. Interestingly, siblings born of a different mother does not appear to be related to respondents’ sexual preferences. Only a boy’s own pregnant mother seemed to leave a sexual imprint.
Freud’s “oedipal phase,” from about 3 to about 5-6 years of age, only overlaps partially with the sensitive period suggested by this study’s data, the researchers are careful to point out. Sexual imprinting is different in that it’s motivated not by sexual drive but because the individual learns what’s normal during a sensitive phase of development and later seeks sexual partners that resemble his (or her) own parents.
What does this mean for women who are pregnant or plan to be pregnant? It means you may be able to predict how attracted your partner will be to you in late pregnancy. Does he have sibling born within five years after him? If so, he’s likelier to be turned on by your pregnant self.
As for the guy I met in the dairy aisle, I’d wager he had a younger brother or sister. I’d bet more on getting this right than the winner of the next World Cup.
*If you like this blog, click here for previous posts and here to read a description of my most recent book, Do Gentlemen Really Prefer Blondes?, on the science behind love, sex, and attraction. If you wish, check out my forthcoming book, available October 11, Do Chocolate Lovers Have Sweeter Babies?: The Surprising Science of Pregnancy.