Saturday, December 24, 2011

Embryo transfer vs implantation

Lots of IVF patients are not very clear about the medical details of their IVF cycle. This sometimes causes them to use medical terms inappropriately. Many are confused about the difference between embryo transfer and embryo implantation and will sometimes use these terms inter-changeably.

There's actually a world of a difference between the two !

Embryo transfer is a simple clinical procedure where the doctor transfers your embryos into your uterus, using a plastic tube called an embryo transfer catheter. This is a mechanical step, which involves transferring the uterus from the petri dish in the incubator to your uterus.

Implantation , on the other hand, is a complex biological process which occurs within the uterus.

How are the two connected ?

When the doctor transfers your embryos on Day 3, they are about 8-cells. They continue to divide in utero , if all goes well , until they form blastocysts. This hatches, and the cells of the embryo embed themselves into the wall of the uterus . This is usually on Day 6 or Day 7 after the egg collection.

Implantation is a silent process over which we have no control. It involves 3 phases - apposition; adhesion ; and invasion, based on an interchange of a beautifully synchronised exchange of molecular signals between the endometrium and the embryo. All the steps are equally important, and need to occur sequentially. There is no special day or time which is more important than another ! We are not able to track or monitor implantation ; and we cannot influence it either. We do give you estrogen and progesterone to prepare your uterine lining for the implantation, but whether or not your embryos will implant; and how many will implant , is not something we can influence or predict.

This is hardly surprising. After all, the embryo is just a microscopic ball of cells - how can we track what it's doing after we have transferred it into your uterus ?

So why don't all transferred embryos implant ? This is simply because human reproduction is notoriously inefficient - after all, not every fertile couple gets pregnant every time they have unprotected sex during their fertile period ! The commonest reason for failed implantation is a genetic abnormality in the embryos , and this is Nature's defense mechanism, to prevent the birth of an abnormal baby. While these defects are often random, they are commoner in older women. This is because the eggs of older women have more genetically abnormalities, because they have "aged" and have genetic defects, which cannot be screened for.

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