IVF may increase placenta risk.
Norwegian researchers say they have found evidence techniques in assisted reproduction may be linked to placenta praevia. This occurs when the placenta covers the cervix and it can harm mother and baby.
Many women are told they have a low-lying placenta but the danger occurs if the placenta does not move up in time for the birth. A Caesarean is usually needed.
The study, in Human Reproduction, found IVF patients were up to six times more likely to develop the condition than those who conceived naturally, an increase from three to 16 in 1,000 births. The study, of 845,384 pregnancies, is the most extensive and the first to make a direct link with IVF techniques.
Researchers say delivering the embryo into the uterus via a catheter might cause uterine contractions.
This could be due to the release of prostaglandin hormones after stimulation of the cervix, leading to more embryos implanting low in the uterus.
Current practice also favours implanting embryos low as this may improve the implantation rate.
A spokesman for the Human Fertilisation and Embryology Authority (HFEA), said: "We always closely monitor developing research in assisted reproduction, but it is important to respond to the broad direction of scientific progress rather than make sudden changes on the basis of individual studies."