A rising proportion of IVF embryos is frozen and transferred later. Those thawing cycles, however, should ideally still involve an ovulation, because the yellow body (which results from ovulation) secretes important substances. This is why we have adapted the organisation of our thawing cycles to the progress of science, which, in turn, gave another significant boost to our success rates!
Section of a human ovary with fully developped yellow body (corpus luteum, top left, marked in blue) and old yellow body turning into white fibrous tissue (corpus albicans, bottom right, marked in green).
The yellow color stems from accumulated carotenoids, like in the macula of the retina.
Thanks to markedly improved freezing techniques (vitrification, since 2013 standard in our laboratory), embryos can be transferred later without loss of quality. This makes sense to prevent hyperstimulation, to allow for pre-implantation testing (for which our laboratory has become a hot spot), or to try for a second or third baby with stored embryos. Worldwide, the majority of such thawing cycles is performed in a programmed fashion, i.e. a suitable uterine lining is build up with external estrogens and progesterone without ovulation (and, therefore, without yellow body). Thus the date of embryo transfer can be set in advance, which is extremely popular among treated couples and allows for equilibrating laboratory workload.
Programmed thawing cycles are practical for everybody involved and produce very high pregnancy rates. Since 2020, however, scientific reports are emerging that both birthweight and preeclampsias (pregnancy-induced hypertension) increase after programmed cycles. The frequency of preeclampsias might more than double, from 3-4% to 9-10%. A possible reason is that in the absence of a corpus luteum, certain blood vessel-widening (vasodilating) substances like relaxin and prorenin are not being produced, despite the fact that they would help to transform the maternal blood circulation in early pregnancy. A cause-and-effect relationship has not been proven yet, but important opinion leaders in reproductive medicine recommend the use of programmed thawing cycle only in women who cannot ovulate.
From July 2022 onward, we have planned thawing cycles with a very mild (so-called monofollicular) ovarian stimulation. The preceding menstruation will be timed with an oral contraceptive or a progestogen pill as usual; this will allow the transfer date to mostly fall in a predefined time window of four days. The actual date of embryo transfer will be set one week in advance, when the ovarian follicle is ready to ovulate. The couples treated by us accept this modification given the clear medical benefits.