Natural cycle seems as effective as hormones for frozen embryo transfers

Using women's natural ovulation cycles for IVF frozen embryo transfers is as effective as a hormonally medicated cycle, and may reduce pregnancy complications, research suggests.

When transferring embryos created by IVF, the endometrium (womb lining) must be sufficiently thick for the embryo to implant. This can either be achieved by tracking a woman's natural ovulation cycle or by using hormonal medication. A clinical trial in China is the first to produce datasets large enough to reliably compare the effectiveness and safety of these two approaches.

'The findings of this study suggest that a natural ovulation regimen for endometrial preparation before frozen embryo transfer is as effective as a programmed regimen in terms of a healthy live birth and results in a lower risk of maternal complications during pregnancy', said the authors in their paper, published in the BMJ.

The randomised trial was conducted across 24 fertility centres, and 4376 women aged between 20 and 40 participated. Half of the women tracked their natural ovulation and half were hormonally medicated, with all participants scheduled to undergo a single frozen embryo transfer. Results showed that healthy live birth rates were similar between the two groups: 41.6 percent for tracked natural cycles and 40.6 percent for medicated cycles.

However, the risk of pre-eclampsia – a serious condition during pregnancy involving high blood pressure – was reduced for the natural ovulation group (2.9 percent compared to 4.6 percent for the hormonally medicated group). Further, the natural ovulation group had fewer cases of early pregnancy loss, caesarean sections, postpartum haemorrhage and placental accreta spectrum (where the placenta fails to easily detach during birth).

Dr William Buckett, an obstetrician and gynaecologist at McGill University in Montreal, Canada, who was not involved in the research, told New Scientist the work was 'the randomised-controlled trial we have been waiting for.'

While acknowledging limitations in the reach of the study to investigate additional maternal complications, the authors wrote that it 'provides proof of concept that an intervention started before conception can lower maternal morbidity later in pregnancy.’

Professor Wei and her colleagues say future research will focus on examining biological markers in the blood and uterus – as well as interactions between the mother and fetus – to better understand the mechanisms behind their findings.