I have been through several IVF attempts using both fresh and frozen embryos, the last being about a year and a half ago. I am 37, and my husband is 40. I am just wondering if there has been any recent advancement in the area of getting the embryos to "stick" to the uterus. That is where the problem seems to be for me, and for many other women also.

M.K.

The issue of getting the embryos to "stick" is a bit more complicated than your question suggests. The process of implantation is complex, and myriad factors can affect the outcome.

The first thing that must happen is contact between the hatched blastocyst (embryo) and the surface of the endometrium (uterine lining). If the growing blastocyst does not hatch out of its "shell" on the sixth or seventh day, the embryo can't attach to the endometrial surface. Why some embryos hatch and other don't is not clear. The women's age, the choice of hormonal stimulation method, and the level of the hormone FSH may all play a role in this first step. In selected cases, a technique called assisted hatching may improve the ability of the embryo to break out of its shell.

Even if the blastocyst hatches properly, the state of the endometrium itself may make a difference. We know that a woman's hormone levels during IVF are dramatically higher than those seen during naturally occurring menstrual cycles. These higher levels can adversely affect the uterine lining, hampering implantation. This may partly account for the higher implantation rate seen following egg donation cycles in which the recipient woman undergoes a drug stimulation regimen that results in more normal levels of both estrogen and progesterone. Perhaps exposing the uterine lining to more appropriate hormone levels makes it more conducive to embryo implantation.

Once the embryo is attached, its next step is to secrete enzymes that enable the newly developing placenta to invade into the uterine lining and grow new blood vessels that exchange nutrients with the maternal blood supply. The necessary steps in this process are only now becoming clear, and studies have not yet determined why and where failure may occur. Nor have researchers determined what tests indicate a potential problem, and -- if a problem is identified -- how it might be corrected.

Unfortunately, this lack of tools to accurately predict implantation failure or determine its cause leaves us little to do other than to look at statistical information. Compiling information from a large number of cycles, we can conclude that if both egg quality and ultrasound appearance of the uterine lining appear normal, then the pregnancy rate per cycle of IVF should remain constant for up to five cycles for fresh embryo transfers. If there is indication of poor egg quality, or if you have already completed four or five cycles, you may wish to consider egg donation.