What is the cause of repeated miscarriages in women over age 35?

A study in the September 2010 issue of Fertility & Sterility examines this questions and comes up with the answer, "It's the chromosomes, stupid." My apologies to Bill Clinton for paraphasing his remark about the economy.

They looked at women age 35 or older who had three or more miscarriages. Among 43 recurrent pregnancy loss (RPL) patients, there were 50 miscarriages in which cytogenetic analysis was performed. In the RPL group, the incidence of chromosomal abnormalities in the POC was 78% (39 out of 50.) That's compared with a 70% incidence (98 out of 140) in the sporadic losses. Thrombophilia results in the RPL patients were normal in 38 patients, four patients had antiphospholipid (APA) syndrome, and one had protein C deficiency. Forty out of 43 had normal uterine cavities. Both thyroid stimulating hormone (TSH) and parental chromosome analyses were normal in all of the patients tested. When the evaluation of RPL included karyotype of the POC, only 18% remained without explanation. However, without fetal cytogenetics, 80% of miscarriages would have been unexplained.

They concluded that in older patients with RPL, fetal chromosomal abnormalities are responsible for the majority of miscarriages. Other causes were present in only 20% of cases.

So, it's the chromosomes.