There will likely be a bit of press this week on a study presented at the Society for Maternal Fetal Medicine that found lower rates of congenital anomalies at second trimester ultrasound screening among mothers over age 35 compared to their younger counterparts.
[The] group looked at the routine second trimester ultrasound screenings of 76,156 euploid fetuses over the course of 18 years at Washington University Physicians, and the data were split into younger than 35 and 35 and older.
Overall, 2.4% (1,804) of those screened had a major anomaly. But contrary to previous beliefs, only 1.7% of women of advanced maternal age had a fetus with a major anomaly compared with 2.6% of younger women (aOR 0.59, 95% CI 0.52-0.66,P<0.001).
The study was well done, but the headlines about it – Older Maternal Age Tied to Lower Risk of Fetal Anomalies – are misleading. That’s because the study did not look at all women over age 35, just those carrying infants with normal chromosomes who made it to the second trimester and presented for a sonogram.
Risks of chromosomal abnormalities increases with maternal age
One of the biggest risks of advancing maternal age is abnormalities in chromosome number, the most common of which are Down’s syndrome (trisomy 21), as well as trisomies of chromosome 18, 13 and abnormalities in sex chromosome number. Fetuses with abnormal number of chromosomes have multiple congenital anomalies of the heart, genitourinary, brain and GI tract.
The risk for trisomy increases from around 1 in 500 at age 22 to 1 in 200 at age 35 , 1 in 65 at age 40 and 1 in 20 at age 40. Those risks are probable an underestimate, since the majority of a fetuses with abnormal chromosome numbers will miscarry early in pregnancy, an explanation for the higher miscarriage rates in older mothers.
Most miscarriages occur in the first trimester of pregnancy, which is why a study that only looks at pregnancies in the second trimester will miss a low of abnormal pregnancies. If you then confine your study further to only those pregnancies with normal chromosome number, you’ve eliminated the majority of infants with congenital anomalies.
Bottom line
This study is not adding much to what we already know about pregnancies in older women, other than to tell us that pregnancies in older women who have had normal chromosomal screening and make it into the second trimester are in general going to do well. Even better in some ways that that of a younger woman.
Think of it as survival of the fittest. The older woman’s fetus has had a harder row to hoe and made it this far.
It is one tough little cookie.
I always knew I was very lucky to have a perfectly normal son at the age of 44 with no fertility treatments (sperm donor) but now I feel even luckier! I did lose a “twin” at about 4 weeks with the same pregnancy.