The news that Britain is set to become the first country to authorize IVF using genetic material from three people—the so-called ‘three-parent baby’—has given rise to (very predictable) divisions of opinion. On the one hand are those who celebrate a national ‘first’, just as happened when Louise Brown, the first ever ‘test-tube baby’, was born in Oldham in 1978. Just as with IVF more broadly, the possibility for people who otherwise couldn’t to be come parents of healthy children is something to be welcomed.
The post The third parent appeared first on OUPblog.
By David Haig
A woman who gives birth to six children each with a 75% chance of survival has the same expected number of surviving offspring as a woman who gives birth to five children each with a 90% chance of survival. In both cases, 4.5 offspring are expected to survive. Because the large fitness gain from an additional child can compensate for a substantially increased risk of childhood mortality, women’s bodies will have evolved to produce children closer together than is best for child fitness.
Sleeping baby by Minoru Nitta. CC BY 2.0 via Flickr.
Offspring will benefit from greater birth-spacing than maximizes maternal fitness. Therefore, infants would benefit from adaptations for delaying the birth of a younger sib. The increased risk of mortality from close spacing of births is experienced by both the older and younger child whose births bracket the interbirth interval. Although a younger sib can do nothing to cause the earlier birth of an older sib, an older sib could potentially enhance its own survival by delaying the birth of a younger brother or sister.
The major determinant of birth-spacing, in the absence of contraception, is the duration of post-partum infertility (i.e., how long after a birth before a woman resumes ovulation). A woman’s return to fertility appears to be determined by her energy status. Lactation is energetically demanding and more intense suckling by an infant is one way that an infant could potentially influence the timing of its mother’s return to fertility. In 1987, Blurton Jones and da Costa proposed that night-waking by infants enhanced child survival not only because of the nutritional benefits of suckling but also because of suckling’s contraceptive effects of delaying the birth of a younger sib.
Blurton Jones and da Costa’s hypothesis receives unanticipated support from the behavior of infants with deletions of a cluster of imprinted genes on human chromosome 15. The deletion occurs on the paternally-derived chromosome in Prader-Willi syndrome (PWS). Infants with PWS have weak cries, a weak or absent suckling reflex, and sleep a lot. The deletion occurs on the maternally-derived chromosome in Angelman syndrome (AS). Infants with AS wake frequently during the night.
The contrasting behaviors of infants with PWS and AS suggest that maternal and paternal genes from this chromosome region have antagonistic effects on infant sleep with genes of paternal origin (absent in PWS) promoting suckling and night waking whereas genes of maternal origin (absent in AS) promote infant sleep. Antagonistic effects of imprinted genes are expected when a behavior benefits the infant’s fitness at a cost to its mother’s fitness with genes of paternal origin favoring greater benefits to infants than genes of maternal origin. Thus, the phenotypes of PWS and AS suggest that night waking enhances infant fitness at a cost to maternal fitness. The most plausible interpretation is that these costs and benefits are mediated by effects on the interbirth interval.
Postnatal conflict between mothers and offspring has been traditionally assumed to involve behavioral interactions such as weaning conflicts. However, we now know that a mother’s body is colonized by fetal cells during pregnancy and that these cells can persist for the remainder of the mother’s life. These cells could potentially influence interbirth intervals in more direct ways. Two possibilities suggest themselves. First, offspring cells could directly influence the supply of milk to their child, perhaps by promoting greater differentiation of milk-producing cells (mammary epithelium). Second, offspring cells could interfere with the implantation of subsequent embryos. Both of these possibilities remain hypothetical but cells containing Y chromosomes (presumably derived from male fetuses) have been found in breast tissue and in the uterine lining of non-pregnant women.
David Haig is Professor of Biology at Harvard University. he is the author of “Troubled sleep: Night waking, breastfeeding and parent–offspring conflict” (available to read for free for a limited time) in Evolution, Medicine, and Public Health. The arguments summarized above are presented in greater detail in two papers that recently appeared in Evolution, Medicine, and Public Health.
Evolution, Medicine, and Public Health is an open access journal, published by Oxford University Press, which publishes original, rigorous applications of evolutionary thought to issues in medicine and public health. It aims to connect evolutionary biology with the health sciences to produce insights that may reduce suffering and save lives. Because evolutionary biology is a basic science that reaches across many disciplines, this journal is open to contributions on a broad range of topics, including relevant work on non-model organisms and insights that arise from both research and practice.
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The post Breastfeeding and infant sleep appeared first on OUPblog.
By Allen J. Wilcox
On making boy babies, and other pregnancy myths
In her novel, Prodigal Summer, Barbara Kingsolver celebrates the lush fecundity of nature. The main character marvels at the way her ovulation dependably comes with the full moon.
It’s a poetic image – but is there any evidence for it?
Actually, no. It’s true that the length of the average menstrual cycle is close to the length of the lunar cycle. But like so many notions about fertility, an effect of the moon on ovulation is just a nice story. The menstrual cycle is remarkably variable, even among women who say their cycles are “regular.” This is not surprising – unlike the movement of stars and planets, biology is full of variation. The day of ovulation is unpredictable, and there is no evidence (even in remote tribal cultures) that ovulation is related to phases of the moon or other outside events.
We humans are susceptible to myths about our fertility and pregnancy. These myths also invade science. One scientific “fact” you may have heard is that women who live in close quarters synchronize their menstrual cycles. The paper that launched this idea was published forty years ago in the prestigious journal Nature1. Efforts to replicate those findings have been wobbly at best – but the idea still persists.
Another scientific myth is the notion that sperm carrying the Y male chromosome swim faster than sperm carrying the X female chromosome. It’s true that the Y chromosome is smaller than the X. But there is no evidence that this very small addition of genetic cargo slows down the X-carrying sperm. As often as this idea is debunked, it continues to appear in scientific literature – and especially the literature suggesting that couples can tilt the odds towards having a baby of a particular sex.
Choosing your baby’s sex
Many couples have a definite preference for the sex of their baby. The baby’s sex is established at conception, which has led to a lot of advice on things to do around the time of conception to favor one sex or the other. Recommendations include advice on timing of sex in relation to ovulation, position during sex, frequency of sex, foods to eat or avoid, etc. The good thing about every one of these techniques is that they work 50% of the time. (This is good enough to produce many sincere on-line testimonials.) Despite what you may read, there is no scientific evidence that any of these methods improves your chances for one sex or the other, even slightly. The solution? Relax and enjoy what you get.
When will the baby arrive?
Everyone knows that pregnancies last nine months – but do they? Doctors routinely assign pregnant women a “due-date,” estimated from the day of her last menstrual period before getting pregnant. The due-date is set at 40 weeks after the last menstrual period. You might think the due-date is based on scientific evidence, but in fact, 40 weeks was proposed in 1709 for a rather flaky reason: since the average menstrual period is four weeks, it seemed “harmonious” for pregnancy to last the equivalent of ten menstrual cycles.
So what are a woman’s chances of actually delivering on her due date? Fifty percent? Twenty percent?
Try four percent. Just like the length of menstrual cycles (and every other aspect of human biology), there is lots of variation in the natural length of pregnancy. If the due-date is useful at all, it is as the median length of pregnancy – in other words, about half of women will deliver before their due-date, and about half after. So don’t cancel your appointments on the due-date just because you think it’s The Day – there’s a 96% chance the baby will arrive some other time.
1. McClintock MK. Menstrual synchorony and suppression.
It’s being said that if you want a baby born on 11-11-11, you should “get ready to get on it this weekend.” So…
By Allen J. Wilcox
You already know where babies come from – the business about sperm and eggs, and getting them together. You also know something about birth control – after all, people spend most of their reproductive years trying NOT to get pregnant.
But there comes a time for many women when they ready to have a baby. That’s when some interesting questions arise.
- Once you stop using birth control, how long does it take to get pregnant?
- Is there something women should do to increase their chances of getting pregnant?
- What can a woman do to help make sure her baby will be healthy?
Let’s start with the last question first. The most important thing a woman can do before getting pregnant is to start taking daily multivitamins with folic acid. Folic acid helps prevent serious birth defects of the brain and spine (neural tube defects) and probably other defects as well. These defects happen very early in the baby’s development – waiting until you think you are pregnant can be too late.
Another thing you can do, if you are a smoker, is to quit smoking. Smoking puts a damper on women’s fertility (although apparently not on the fertility of men – life is not fair). Smoking also increases the small chance of fetal death later in pregnancy. Do yourself (and your baby) a favor, and give up the cigarettes.
Besides that, what should you do (besides the obvious)?
Nothing.
Really, nothing. You already have a lot going for you. Consider the benefits of your family history – not a single one of your ancestors was infertile. If you are a reasonably healthy person with no history of reproductive problems, and if you are having unprotected sex at least weekly, biology is on your side.
Some useful facts
There is a spectrum of fertility, ranging from very low to very high. You won’t know where you are on that spectrum until you actually try to conceive. On average, your chance of getting pregnant in the first month is 25%. For a few unlucky couples, the chances are zero – they are sterile. Other couples may have a 50% or 75% chance of getting pregnant in their very first month of trying. For couples as a whole, about half will be pregnant after three months. That goes up to two-thirds of couples after six months, and more than 90% after a year. Even if you don’t conceive in the first year, you still have a 50% chance in the next year or so. Only about 5% or so of couples are unable to conceive at all by natural means.
Probably the biggest predictor of fertility is woman’s age. Women are at their reproductive peak during their twenties. As they move through their thirties, their fertility begins to decline. This is relevant because many women (for lots of good reasons) delay their childbearing until they are in their 30s or even older. If a woman is not so fertile to start with, this delay can cause problems. Unfortunately, there is no medical test to tell women in advance how fertile they are.
The fertility window
Let’s get down to the biology. Pregnancy happens when couples have sex during the five days before ovulation and the day of ovulation itself. (In other words, sperm can survive up to five days in the woman’s reproductive tract.) This six-day fertility window gives you a fairly wide span of days in each cycle for intercourse that can produce pregnancy.
But there is a catch. Most women don’t know
