06/11/2013 09:29 am ET Updated Aug 11, 2013

Challenging the Fertile Window

We're making babies like it's 1930.

Since the thirties it has been widely accepted that conception will occur only in a "fertile window" around ovulation, roughly midway between one menstruation and the next. The idea is that the rest of the month is "safe." This is the foundation for the rhythm method and other birth-control techniques in which a woman monitors her basal body temperature or the mucus secreted by glands in the cervix (neck of the womb). Yet, despite the confidence with which physicians and fertility experts cite this timeframe, humans regularly have sex and conceive outside of such a limited window. While the mechanics of human reproduction have stayed the same in the past 80-plus years, there is no reason why our view of conception should be stuck in the early 20th century.

The first question to ask is why copulation regularly occurs outside of the so-called safe period, not only in humans but also in apes and monkeys In most mammals, females typically mate only during a clearly defined period of estrus ("heat") lasting about three days, which is what would be expected from the accepted survival times of eggs and sperms. Yet monkeys, apes and humans lack a clear estrus and typically engage in copulation for considerably more than three days.

In my research into human and primate reproduction, I have found evidence for a major challenge to the notion of a fertile window. My starting-point was an unusual finding reported by zoologist Richard Kiltie in 1982, showing that pregnancy lengths in monkeys and apes (higher primates) seem to be about twice as variable as in other mammals. After collecting additional data, I confirmed Kiltie's finding. So, unless pregnancy lengths really are more variable in monkeys and apes than in other mammals -- which is inherently unlikely -- the doubled variation reported for higher primates must be an artifact of the way in which we infer conception time.

In mammals with a clearly defined estrus, conception can generally be pinpointed quite accurately from mating. Variability in pregnancy lengths based on copulation dates generally reflects real variation in the interval between conception and birth. Monkeys and apes are quite different because copulation often occurs on several days of the ovarian cycle and does not precisely indicate conception time. It is taken for granted that copulation leading to conception, like ovulation, typically occurs at mid-cycle. But if copulation at other times can lead to conception, this could explain why pregnancy lengths seem to be more variable than in other mammals. Storage of sperm somewhere in the female reproductive tract could allow a gap of several days between copulation and conception. This leads directly to a testable prediction: Apparent variation of pregnancy lengths for monkeys and apes should decrease when conception is inferred from more reliable evidence to pinpoint ovulation time, such as hormonal monitoring of female cycles. Studies of various higher primates have shown that pregnancy length is, indeed, less variable when determined from ovulation rather than copulation. So sperm storage emerges as a distinct possibility.

Evidence from clinical studies and procedures such as artificial insemination indicate that sperms may be stored for ten days or more, at least in women. Moreover, my analysis of data for over four thousand human pregnancies following single acts of coitus indicates an extensive window for conception in the cycle.

It has long been known that human sperms can survive intact for several days in mucus produced by the cervix. Ironically, a great deal is known about mucus produced by the neck of the womb, particularly because it has often been monitored to identify the fertile window. But remarkably little is known about sperms found in mucus, although it has been observed that abnormal sperms are filtered out and survivors are stored in crypts that produce the mucus. Concrete information is seemingly limited to a single landmark 1980 study by gynecologist Vaclav Insler and colleagues, in which 25 women scheduled for hysterectomy (surgical removal of the womb) bravely volunteered for artificial insemination before surgery. After the womb was removed, microscopic examination of serial sections revealed that sperms colonized crypts along the entire length of the cervix within two hours after insemination. Insler and his team found that up to 200,000 sperms were found stored in the crypts of a single womb. In fact, they also reported that live sperms have been found in cervical mucus up to the ninth day after insemination. Taking the evidence as a whole, they suggested that the cervix serves as a sperm reservoir from which viable sperms are gradually released for migration into the oviduct. Such slow, controlled release could ensure the survival of sperms capable of fertilization over an extended period lasting more than a week.

Physicians rarely interact with veterinarians, so it took my wide-ranging survey of biological evidence to connect up with mounting evidence since 1967 that sperm storage for about ten days occurs in the humble domestic dog. As with monkeys and apes, variation of pregnancy lengths in dogs is about double the mammal norm if calculated from mating dates. But variation is drastically reduced when pregnancy lengths are based on hormonal monitoring of ovulation times, falling in the usual range for non-primates. In the case of dogs, though, microscopic images revealed that sperms seem to be stored mainly in crypts in the womb itself.

At this point you may be asking: why should we care about the existence of a fertile window? In fact it may have a direct impact on the health of your unborn child. When copulation occurs near the beginning or end of a fertile window, there arises the risk of fertilization with timeworn gametes. Studies of laboratory mammals have shown that this is likely to lead to pregnancy loss or fetal deformity -- a finding that is woefully underreported in discussions of human conception and pregnancy.

The possibility of sperm storage opens the way for entirely new interpretations of events leading up to human pregnancy. The notion of a fertile window needs a radical overhaul, not just for humans but also for monkeys and apes.