FAR from being a passive container, the human uterus seems to be highly selective about which embryos it accepts. Women with less 鈥渇ussy鈥� uteruses may be at higher risk of miscarriage.
Miscarriage is the most common complication of pregnancy, with around 1 in 3 embryos lost before 6 weeks of pregnancy and a further 10 per cent before 12 weeks. For years, faulty embryos or problems such as abnormal clotting or immune responses have been blamed.
at Imperial College London wondered if another process was at work. He had noted that many women who had repeated miscarriages claimed to have conceived incredibly quickly. 鈥淓ach one of their pregnancies was conceived within one or two months of trying,鈥� he says. What鈥檚 more, some studies have hinted that embryos implanting outside the normal window of uterine receptivity were more likely to miscarry.
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To investigate further, he and his colleagues took cells from the uteruses of women who had undergone miscarriages and ones who hadn鈥檛. They measured the expression of a key regulator of uterine receptivity called PROK1 and levels of prolactin, a marker of decidualisation 鈥� the monthly process by which the uterus prepares to receive an embryo. Decidualisation involves a thickening of the uterine wall and the growth of new blood vessels.
Expression of PROK1 was higher in the women who had miscarried than in those who hadn鈥檛 and this was maintained for longer, suggesting that their implantation window lasts longer. These women also produced far less prolactin, a sign that their cells don鈥檛 decidualise properly.
Further studies indicated that this impaired decidualisation interfered with the signalling between the embryo and the uterus at the time of implantation. The researchers conclude that these uteruses are less picky, allowing abnormal embryos to implant, which later spontaneously abort (PLoS One, ).
Abnormal decidualisation may also interfere with placental formation 鈥� another reason such women are more likely to miscarry, says Brosens.
These are exciting findings says , a pregnancy researcher at Queen鈥檚 University in Toronto, Canada. 鈥淲e have invested huge effort in assessing the quality of the embryo. This [study] strongly suggests that we must be aware of the endometrial environment as well.鈥�
A test might be developed to predict whether a woman is at high risk of miscarriage before she tries for a baby. With a better understanding of decidualisation, drugs might be developed to reduce miscarriage.
