I am 35 and have had four miscarriages in the past two years. The first two were blighted ovum. The last one was trisomy 12. We know about trisomy 21, but what is trisomy 12? Could this be related to my first two blighted ovum? Last year both my husband and I had chromosomal blood tests done, and results were normal. Is there some way we should check genetically my husband's semen and my eggs?

-- V.C.

Each of us has 46 chromosomes -- 22 pairs of non-sex chromosomes, called autosomes, plus one pair of sex chromosomes (XX or XY). So, you should have two chromosome 12s, two chromosome 21s, etc. In your miscarriage, there was an extra chromosome 12. Although not all trisomies are lethal, this extra chromosomal material often results in miscarriage.

We don't really understand the whys of trisomy. Often it is due to advanced maternal age. That is not your situation. But studies have suggested that if you have one pregnancy loss due to aneuploidy (extra or missing chromosomes), it would appear that other losses will also be due to aneuploidy. For women at age 35, one-third of the eggs seen at IVF are aneuploid (have an abnormal number of chromosomes). Aneuploidy in sperm is infrequent, although the risk may be higher in men exposed to chemotoxins.

While testing may indicate that you or your husband has an increased percentage of aneuploid sperm or eggs, a significant number will still be normal. I would suggest you review your history with a genetic counselor. It's possible that you might benefit from IVF and embryo biopsy so that you can determine whether the embryos are normal before transfer.