IVF may increase the risk of a potentially dangerous complication of pregnancy, Norwegian research suggests.
Embryos may be placed in a vulnerable position
Placenta praevia is a condition in which the placenta covers part or all of the cervix, blocking a baby's passage into the birth canal.
Researchers calculated the risk rose from about three in 1,000 pregnancies in the general population, to 16 in 1,000 for women who had had IVF.
The study of 845,000 cases is detailed in the journal Human Reproduction.
The study, by a team at St Olavs University Hospital, in Trondheim, also found a three-fold higher risk among mothers who had had two pregnancies, once conceiving naturally and once with assistance through IVF, or ICSI, in which a sperm is injected directly into an egg.
The risk rose from seven in 1,000 births for women who had had two natural conceptions, to 20 in 1,000 births for women who had had one natural conception, and one assisted conception.
The researchers took factors such as the age of the mother into account.
Placenta praevia can cause haemorrhaging in the mother, and increases the risk of a premature birth, and problems during delivery.
Small studies have suggested in the past that placenta praevia is more common after the use of assisted fertility techniques.
The Norwegian study was much larger, considering data on over 845,300 pregnancies.
The researchers believe it is the first time an increased risk of placenta praevia has been directly linked to the reproductive techniques used.
Lead researcher Dr Liv Bente Romundstad focused on the 1,349 women in the study who had conceived spontaneously in one pregnancy and after assisted fertility in the other.
"Regardless of whether it was the first or second pregnancy that was conceived through assisted reproductive technology, we found a nearly three-fold higher risk of placenta praevia.
"This suggests that a substantial proportion of the extra risk may be attributable directly to factors relating to the reproduction technology."
The underlying mechanism causing the placenta praevia is not clear.
It is possible that IVF may trigger contractions, leading to embryos implanting lower down the uterus than in natural conceptions.
Alternatively, doctors may position the embryo lower down the uterus in order to improve implantation rates.
The researchers are calling for fertility clinics to record the position of every transferred embryo.
Dr Peter Bromwich, from the Care fertility clinic in Northampton, described the study as "fascinating".
He said: "I have not come across this suggestion before. I already do measure the position of transferred embryos, but I will start to record it too now."
However, he added: "Placenta praevia is a rare condition, and the fact that it might be a little less rare in IVF pregnancies should not be a cause for concern for people having the treatment."
Dr Mark Hamilton, chairman of the British Fertility Society, said: "Patients who are considering IVF treatment should discuss concerns with their gynaecologist in advance of treatment and those who are pregnant might want to discuss this with their obstetrician."
The Human Fertilisation and Embryology Authority said the research would be carefully considered by its Scientific and Clinical Advances Group.