7 January 2012 - The clearest evidence yet to link an embryo’s growth to its risk of pregnancy loss will be presented at the British Fertility Society Annual Meeting in Leeds. The research, by scientists from the University of Nottingham’s School of Clinical Sciences, also challenges the notion that twins grow more slowly in the womb. A means of identifying singleton pregnancies that are at higher risk of pregnancy loss is one potential application of this work.
Researcher Dr Shyamaly Sur and team tested two hypotheses - whether twins grow more slowly than singletons in the womb, and whether restricted growth in the first trimester (from conception until 12-13 weeks) is associated with pregnancy loss. The team tracked the growth of 247 singleton and 264 twin embryos conceived through IVF, allowing them to know the embryos’ precise gestational age. For each embryo, the crown-rump length (CRL; distance from top of the embryo’s head to the bottom of its buttocks) was measured once during the first trimester from a transvaginal ultrasound scan and the pregnancy then monitored until birth. Using the CRL measurements, the researchers developed two growth curves to test their hypotheses: one to record the growth of singleton embryos and one for twin embryos. Sensitivity analysis was used to analyse whether embryos that went on to miscarry were growth restricted (defined as being in the smallest 5% of the normal sample; often called the 5th Centile).
In pregnancies resulting in a live birth, twin embryos grew at the same rate as singletons during the first trimester (F test, F=2.66, P=0.0713). For singleton pregnancies, growth restriction in the first trimester was a sensitive predictor of subsequent pregnancy loss. 77.8% (34/44) singleton pregnancies that miscarried were growth restricted, whilst 98.1% (209/213) of singleton pregnancies that did not miscarry were not growth restricted. This was not the case in twin pregnancies where only 28.6% (12/42) of pregnancies that miscarried were growth restricted although a similar number (98.5%: 193/196) of twin pregnancies that did not miscarry were not growth restricted.
The data show that we can potentially identify singleton pregnancies at higher risk of pregnancy loss through accurate measurement of growth during the first trimester. The next step in this research is to attempt to predict pregnancy loss prospectively using the 5th Centile for growth and also examine the relationship between restricted growth and pregnancy loss in more detail to elucidate the reasons behind this relationship.
Researcher Dr Shyamaly Sur, Subspecialty Trainee in Reproductive Medicine at the School of Clinical Sciences, based at Nottingham University Hospitals NHS Trust, said:
“Although it has been widely accepted that twin embryos exhibit slower growth in early pregnancy, this hypothesis has not been tested before. Our work shows that twin embryos actually grow at the same rate as singleton embryos in the early stages of pregnancy.
“Our study also provides the clearest evidence yet that restricted growth in single embryos during the early stages of pregnancy is related to subsequent pregnancy loss in that pregnancy. Our method of following women who have undergone IVF treatment meant that we were able to accurately date the pregnancies and therefore were able to relate their subsequent growth to the date of conception very precisely. This is not possible in spontaneous pregnancies as the exact date of conception is never known and simply estimated from the date of the last menstrual period.
“We hope that this work will go towards developing a new system to identify pregnancies at risk of pregnancy loss by using evidence of early growth restriction as a factor. More research is now needed to investigate the relationship between growth and the underlying causes of pregnancy loss in more detail. At NURTURE we are focussing on how blood flow to the womb lining and embryo quality influence conception rates and subsequent pregnancy loss.”
The British Fertility Society is a national multidisciplinary organisation representing professionals practising in the field of reproductive medicine. The British Fertility Society is committed to promoting good clinical practice and working with patients to provide safe and effective fertility treatment.
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