Why Embryos Are in a Hurry

Scientists have shed some light on the mysterious winnowing process inside a woman's body that sorts good embryos from bad. A study in this week's New England Journal of Medicine suggests that embryos that take longer to implant in the womb have a lower chance of surviving.

The work builds on a study in which epidemiologist Allen Wilcox of the National Institute of Environmental Health Sciences in Research Triangle Park, North Carolina, found that about a quarter of all human embryos don't survive the first 6 weeks after conception. Searching for an explanation, his team wondered whether there is a connection between the time it takes an embryo to implant in the womb's lining (the endometrium) and the likelihood of miscarriage.

The team recruited 221 women aged 25 to 35 who were attempting to conceive and collected daily urine samples from the time they stopped using birth control until 8 weeks after they became pregnant (or for 6 months, if they failed to get pregnant). By monitoring levels of estrogen and progesterone, which drop and rise, respectively, when a woman ovulates, they determined the time of ovulation, down to the day. Implantation is signaled by chorionic gonadotropin (HCG), a hormone that embryos start churning out once they take root in the endometrium.

Of 189 pregnancies, 102 embryos implanted in the first 9 days after conception; of these, only 13 (13%) ended in a miscarriage. But the failure rate rose to 26% for implantation on day 10, 52% on day 11, and 82% for embryos that implanted even later. Wilcox speculates that the uterus is receptive to embryos for a limited time, then tries to filter out ones that, because of genetic problems or other reasons, take too long to travel from inside the ovarian tubes--where conception usually takes place--to the endometrium. If this scenario is true, Wilcox says, it may be a bad idea to hormonally tinker with the uterus to prolong its receptive period, as some scientists have proposed; that, he says, could increase the chances of defective embryos implanting.

Another explanation for the findings could be that the day HCG appears in the urine doesn't signal the embryo's implantation, but rather its ability to secrete HCG. Embryos of poorer quality may produce less of the hormone, making it show up later in measurements, says Mary Lake Polan, chair of the department of gynecology and obstetrics at Stanford University School of Medicine. The study is a clear call for more research into the implanting embryo, she says: "They perceive everything from the perspective of the endometrium. ... But that's only half of it." Next, researchers will have to "actually examine what the embryo is doing," says Polan.