The researchers say that this is the first time the effect of maternal height on multiple implantation has been shown. "In natural dizygotic twinning it is not possible to distinguish between multiple ovulation and implantation," said Dr. Lambers "whereas by studying pregnancies in mothers who have undergone DET we know exactly how many embryos were transferred and can therefore draw firm conclusions about the factors that influence the development of twin pregnancies."
The researchers studied data from the 19840 women in The Netherlands who underwent IVF treatment between 1983 and 1995 (the OMEGA study group). They selected all first fresh (unfrozen) IVF and ICSI cycles using DET that resulted in the delivery of a single or twin baby, both living and stillborn. Excluded from the analysis were cases where there were insufficient data from medical files, patients who used donated oocytes or who had undertaken fertility treatment besides IVF and ICSI, those who had had an abortion, an ectopic pregnancy, or a miscarriage. The remaining group was analysed using a dependent variable -- whether or not the pregnancy was single or multiple -- and a number of independent variables: BMI, weight, height, maternal age, number of oocytes retrieved, use of alcohol, level of education and parity.
Of the 6589 patients who completed their first IVF cycle, 2357 women had DET, resulting in 371 singleton and 125 twin pregnancies. "When we came to analyse the twin group, we found that a maternal height of over 1.74cm and more than 11 oocytes retrieved were independently associated with twin pregnancies," said Dr. Lambers.
"The positive relationship between a higher number of oocytes retrieved and a twin pregnancy probably reflects the fact that these women would have had a larger choice of good quality embryos. But the association between increased height and multiple pregnancy is more difficult to explain. Previous studies have shown that women who are prone to conceive twins after IVF have a higher level of vascular endothelial growth factor-A (VEGF-A), which promotes the growth of blood vessels at the site of implantation. Perhaps there is also an association between tall stature and increased VEGF-A levels," she said.
"Doctors working in IVF are faced with a constant balancing act between giving the best chance of achieving a pregnancy without incurring all the problems associated with multiple births," said Dr. Lambers. "We hope that our work could be used in future as a basis for predictive models that assist in the choice between single and double embryo transfer. This is an important topic, since multiple pregnancies involve a higher risk of problems for the mother, such as pre-eclampsia, and also for the baby due to pre-term or immature delivery."
Materials provided by European Society of Human Reproduction and Embryology (ESHRE). Note: Content may be edited for style and length.
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