I have been in an IVF program since 1995. My husband and I started with 21 embryos, and now we are down to five. My last transfer ended with a positive result, with HCG levels at 50 at two days, but later at 0. What are the chances that the same thing will happen again? I often wonder if something's killing the embryos. My doctor says that my uterus is healthy and perfect. So what could be the problem?

-- Judy

It is often difficult to know why someone does not conceive with IVF frozen embryo transfer. Many things have to happen correctly in order to conceive. First, the embryos need to be healthy before freezing. The more cells and the more normal the embryo is when frozen, the greater its potential to survive the freezing and to implant. In general, 50-65 percent of embryos will survive the freezing-thawing process.

Other factors include your age. The younger you are, the more likely the process will be successful. Your endometrium (uterine lining) may have looked normal on ultrasound, but we are only beginning to learn what happens after we transfer the embryos into the uterine cavity and which factors may interfere with implantation. Some studies are examining whether the immune system plays a role in IVF failures. While early studies suggest testing and treatment may be of benefit, it is just too early to recommend uniform testing and treatment. The data are not yet conclusive, and treatment may pose considerable risks.

I would recommend that you meet with your physician to determine whether you should have additional testing to give you the best odds of success in a future embryo transfer cycle.