BASIC BOOKS, JUNE 2013 Most women are taught that conception only occurs if coitus takes place within a so-called “fertile window” around ovulation, usually halfway through a menstrual cycle. This underlies the “rhythm method” and related birth-control techniques that include monitoring a woman’s basal body temperature or mucus secreted by glands in her cervix. It has long been held that the window lasts only 3 days, allowing 2 days for survival of ejaculated sperm and at most a day for egg survival after ovulation.
But pregnancy lengths in higher primates (monkeys, apes, and humans) seemingly vary twice as much as in other mammals. Why?
It is well known that female bats can store sperm for months between copulation and fertilization. But sperm storage has rarely been considered for primates. Intervals between copulation and birth vary more if sperm are stored in the female’s body. It has also been shown that human sperm can survive for at least 5 days in the uterus, but no attempt has been made to identify an upper limit. I explore the implications of possible sperm storage and other aspects of the science of sex in my new book, How We Do It: The Evolution and Future of Human Reproduction.
In most mammals, females mate only during a clearly defined estrus period lasting about 3 days, matching survival times of eggs and sperm. But monkeys and apes, like humans, have no obvious estrus and typically mate on many days, not just near ovulation.
Many ingenious hypotheses have been suggested to account for copulation when ovulation is unlikely. Yet all completely ignore a fundamental problem: copulation near the beginning or end of a fertile window poses the danger of fertilization with a timeworn sperm or egg. Studies of laboratory mammals have shown that this triggers pregnancy loss or fetal deformity. So why do humans and other higher primates copulate several days away from ovulation?
Variation in pregnancy lengths yields a clue. Because mammalian pregnancies generally show precise timing, it is unlikely that higher primate pregnancies really vary. Apparently doubled variation in monkeys, apes, and humans is presumably an artifact of how conception dates are inferred.
Pregnancy durations for monkeys and apes are mainly calculated from observed copulations. In mammals with an obvious estrus, this generally pinpoints conception dates accurately, and variation in pregnancy lengths corresponds to actual conception-to-birth intervals. But because higher primates often copulate on several days of the cycle, conception time cannot easily be detected. It is assumed that both ovulation and copulation resulting in conception typically occur at mid-cycle. But sperm storage in the female could introduce a delay of several days between mating and conception.
We can test this novel hypothesis by determining whether variation of pregnancy lengths for monkeys and apes decreases when conception is inferred more reliably, by detecting ovulation time hormonally. Studies of various species have confirmed that pregnancy lengths based on ovulation do vary less.
Although the possibility of sperm storage has rarely been considered, procedures such as artificial insemination indicate that sperm may survive in a woman’s body not just for 5 days but for 10 days or more. Moreover, human pregnancies following single coitus indicate conception virtually throughout the cycle.
Concrete medical information is seemingly limited to one landmark 1980 study led by Israeli gynecologist-obstetrician Vaclav Insler. Twenty-five women bravely volunteered for artificial insemination on the day before hysterectomy. Examination then revealed up to 200,000 sperm cells in the mucus-producing crypts of the cervix. Insler and colleagues suggested that the cervix serves as a reservoir from which viable sperm are gradually released. Medical scientists never followed up on this discovery.
The possibility of sperm storage opens the way for entirely new interpretations of human conception. And the notion of a fertile window needs radical revision, not just for humans but also for monkeys and apes.
Robert Martin is the A. Watson Armour III Curator of Biological Anthropology at the Field Museum in Chicago, and a member of the Committee on Evolutionary Biology at the University of Chicago. Read an excerpt of How We Do It.