A study found that while fresh embryo transfers also share a similar risk of developing these types of disorders, the risk increases by a significant 74 per cent when frozen embryos are used.
The findings, published in the journal Hypertension, compared the risk of a woman developing high blood pressure (hypertensive) disorders such as preeclampsia when using frozen embryo transfers verses fresh embryo transfers – where the fertilised egg is implanted immediately – as well as natural conception.
Pregnant women are more prone to hypertensive disorders because of the extra strain on their heart and blood vessels.
Such disorders, including preeclampsia superimposed on chronic hypertension, gestational hypertension, and chronic hypertension, affect around eight to 10 per cent of all pregnant women.
They can be associated with dangerous complications for both the mother and the baby. Unmonitored and untreated preeclampsia can lead to fatalities, with more than 70,000 maternal deaths and 500,000 fetal deaths occurring as a result of the disorder globally each year.
Dr Sindre H Petersen, the study’s lead author and researcher at the Norwegian University of Science and Technology, said: “In summary, although most IVF pregnancies are healthy and uncomplicated.
“This analysis found that the risk of high blood pressure in pregnancy was substantially higher after frozen embryo transfer compared to pregnancies from fresh embryo transfer or natural conception.”
Scientists analysed data from nearly 2.4m women aged 22 to 44 across medical birth registries from Denmark, Norway and Sweden.
The data included those who had both an IVF pregnancy and a naturally conceived pregnancy.
Of the 4.5m pregnancies that resulted in single deliveries, 4.4m were naturally conceived, around 78,000 were fresh embryo transfers, and around 18,000 were frozen embryo transfers.
More than 33,000 pregnancies were grouped for “sibling comparisons”, which only involved mothers who conceived both naturally and using IVF.
In mothers who had both a natural conception and a frozen embryo transfer, the study found that the risk of hypertensive disorders in pregnancy after the latter was twice as high compared to the former.
However, researchers said they were unable to pinpoint what part of the frozen cycle or frozen transfer may contribute to the higher risk.
Dr Petersen said: “Our sibling comparisons indicate that the higher risk is not caused by factors related to the parents, rather, however, that some IVF treatment factors may be involved.
“Future research should investigate which parts of the frozen embryo transfer process may impact risk of hypertension during pregnancy.”
He added: “Our results highlight that careful consideration of all benefits and potential risks is needed before freezing all embryos as a routine in clinical practice.
“A comprehensive, individualised conversation between physicians and patients about the benefits and risks of a fresh versus frozen embryo transfer is key.”
Additional reporting by PA