You have had your egg retrieval and are now waiting for your embryo transfer. Instead of having your transfer 5 days later, you are told that the embryos are “dividing slowly” and the transfer is postponed or cancelled. On day 6 finally, blastocysts are formed and you are recommended to freeze the embryos instead of having a transfer.
It is natural to feel disappointed, and we do know how anxious you are to get pregnant. However, there is a very good reason why freezing slow developing embryos and transferring them in a subsequent programmed cycle makes sense.
During a natural cycle, a dominant follicle develops in the ovary in response to certain hormones. This follicular growth, in turn, cause a rise in estrogen. This rise in estrogen triggers ovulation and the start of progesterone production by the ovaries.
Once progesterone has begun to be produced by the corpus luteum (the ovulated follicle), a series of changes begin to take place within the endometrial lining, preparing the lining for implantation. Since the onset of progesterone production is tied to ovulation, embryos and the endometrial lining develop at a perfectly timed rate. This “window of endometrial receptivity” is short, and if the embryo and the lining are out of sync, pregnancy cannot result because the embryo will not recognize the uterine lining.
Embryos should reach the blastocyst stage by day 5 of development. If the embryo is developing slowly, not reaching the blastocyst stage until day 6, the “window of opportunity’ for implantation might be missed. Due to implantation being such a highly complex and orchestrated interaction between the lining of the uterus and the cells of the blastocyst, transferring embryos with embryo slow development reduces the likelihood of pregnancy occurring.
So, why freeze a slowly developing embryo?
Just because an embryo is developing slowly, it does not necessarily mean anything is wrong with the embryo, it just means that the embryo and the uterus cannot recognize each other. As long as the embryo is a blastocyst by day 6, that embryo can be frozen and transferred during a frozen embryo transfer.
Blastocysts are frozen using a process called vitrification, and we see about 98% survival at the time of thaw. The implantation rate of these vitrified blastocysts are the same, and sometimes even higher, than the fresh transfer. This is due to the precise synchrony between the endometrial lining and the developmental stage of the thawed embryos. During a frozen embryo transfer, no follicles are developed and there is no endogenous production of progesterone to throw off the window of implantation.
Although we know that when you do in vitro fertilization (IVF) you want your transfer immediately, ultimately, waiting to get things just right might be the best way to help you achieve your dreams of pregnancy.
Liza has been an embryologist at InVia Fertility Specialists since 2002. She completed her studies, first in animal science at Southern Illinois University, followed by clinical embryology/reproductive genetics at Eastern Virginia Medical School. Her interests include reproductive genetics, fertility preservation, and mitochondrial function in the human oocyte. She loves being a part of helping others achieve and experience the joy of becoming a parent. Seeing the end result .your baby.makes all the hours in the lab worth it!