A study published Thursday 25 May) in Human Reproduction concluded that there was a six-fold higher risk of the condition in single-baby pregnancies conceived through ART compared with natural conception and a three-fold higher risk among mothers who had conceived both naturally and through ART.
"What this means in absolute terms is that after adjusting for factors such as maternal age, which tends to be higher in ART pregnancies, the risk of placenta praevia rose from about three in 1,000 births to 16 in 1,000 births for ART pregnancies compared with the general population, and from about seven in 1,000 births to 20 in 1,000 births for those who had both assisted and natural conceptions," said lead author Dr Liv Bente Romundstad, from the Department of Obstetrics and Gynaecology at St Olavs University Hospital in Trondheim.
Placenta praevia, where the placenta attaches itself to lowest segment of the uterine wall and either blocks or partially blocks the cervix, is associated with serious and potentially life-threatening problems for mother and baby. These include haemorrhage in the mother, either in before or after the birth, and the need for caesarean section in the case of total placenta praevia. For the baby the risks include prematurity and perinatal problems.
Some small studies have suggested in the past that placenta praevia is more common after ART, but the authors believe that the new research looking at over 845,300 pregnancies in the Norwegian Medical Birth Registry between 1988 and 2002 is the most extensive in the world to examine links between placenta praevia and ART. Furthermore, it is the first to indicate that the increase in risk may be directly related to the reproductive techniques used.
Dr Romundstad explained: "As well as comparing placenta praevia among women who had given birth only to babies conceived through IVF or ICSI and the general population, we studied 1,349 women 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 ART 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. One possibility could be the position that the embryo is placed in the uterus when it is transferred via a catheter through the cervix. The procedure may induce uterine contractions, possibly due to the release of prostaglandins after stimulation of the cervix, leading to more embryos implanting low-down in the uterus. In addition, because research has shown that transferring the embryo to a position low in the uterus may improve implantation rates, current practice tends to favour placing the embryo low down.
Their findings have so concerned the research team that they are calling for ART centres to monitor and record the distance of every transferred embryo from both the internal cervical entrance and the uterine fundus (the part of the uterus lying above the opening of the fallopian tubes).
"We now routinely do this, but we need other centres worldwide to do this as well," said Dr Romundstad. "Although the risk of placenta praevia is considerably higher with ART it is still quite rare, which means it will probably take several thousand pregnancies to get sufficient data to be able to make any definite recommendations about clinical practice."
 Increased risk of placenta previa in pregnancies following IVF/ICSI; a comparison of ART and non-ART pregnancies in the same mother. Human Reproduction. doi:10.1093/hunmrep/del153.
2 IVF – in-vitro fertilisation. ICSI – intracytoplasmic sperm injection: process by which an ooctye is fertilised by injecting a single sperm into the ooctye.
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