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Women who conceive via in vitro fertilization are as much as six times more likely to develop a certain pregnancy complication than women who conceive naturally, says a study out today.
Norwegian researchers analyzed nearly 850,000 single-baby pregnancies reported to their Medical Birth Registry from 1988 to 2002. About 7,600 of the pregnancies were a result of in vitro fertilization, in which eggs are taken from the mother and fertilized in a laboratory. Resulting embryos are then deposited in the uterus.
In the current issue of Human Reproduction, the scientists link in vitro fertilization to an increased risk of placenta previa, a potentially dangerous condition in which the placenta totally or partially covers the cervix, blocking the baby's exit through the birth canal.
Placenta previa, while uncommon, is a leading cause of vaginal bleeding in the second and third trimesters. In the USA, the condition complicates an estimated three to five deliveries per 1,000, or roughly 12,000 to 15,000 per year. More than 80% of babies born to mothers with placenta previa are delivered by cesarean section, which greatly has reduced deaths from this condition.
The causes of placenta previa are not clear. Women over 30, smokers and those who already have delivered several babies are at higher risk, but the scientists accounted for such factors when calculating the risk of placenta previa in the IVF pregnancies vs. the naturally conceived pregnancies.
To make sure they weren't missing any confounding factor, the researchers identified 1,349 women who had conceived both naturally and with IVF. They found that the IVF pregnancies, whether they occurred before or after the naturally conceived pregnancies, were three times more likely to be complicated with placenta previa.
The scientists speculate that inserting embryos into the uterus through the cervix, the final step in in vitro fertilization, may induce contractions, which could cause the embryo to implant low in the uterus, raising the risk of placenta previa. Doctors also often place embryos low in the uterus, because research has shown the practice might improve the chance of a pregnancy.
The authors called for infertility clinics worldwide to measure and record the distance of every transferred embryo from the cervical entrance as well as from the top of the uterus.
Lead author Liv Romundstad, an infertility specialist at St. Olav's University Hospital in Trondheim, emphasized that placenta previa is uncommon, even after in vitro, so a higher risk should not dissuade women from undergoing the procedure.
However, she said, doctors and midwives who provide prenatal care should keep in mind that placenta previa may be more common in pregnancies following in vitro fertilization.
William Gibbons, president of the Society for Assisted Reproductive Technology, called the authors' theories as to why in vitro might increase the risk of placenta previa "reasonable." It is important to continue studying infertility's role in some of the differences between in vitro and naturally conceived pregnancies, Gibbons said.
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