SYDNEY, AUSTRALIA – Short fetal femur length in the second trimester was associated with a significantly increased risk of Down syndrome, trisomy 13 and 18, and unbalanced autosomal structural abnormality in a large population study.
The study of 147,766 Danish singleton pregnancies with a second-trimester malformation scan showed that short femur length – defined as below the fifth percentile – was present in 16.2% of the fetuses affected by trisomy 21 (odds ratio, 10.3).
Dr. Ann Tabor
The data were collected from the Danish National Fetal Medicine database and presented at the International Society of Ultrasound in Obstetrics and Gynecology world congress.
Individuals with trisomy 13/18 also had a significantly higher incidence of short femur length (OR, 7.3), as did individuals with unbalanced autosomal structural abnormality (OR, 23.8).
Researchers also found that pregnancies in which the fetus had a short femur length also were more likely to result in delivery before 34 weeks’ gestation (OR, 4.2) and small-for-gestational-age infants (OR, 4.3).
Dr. Ann Tabor said the first-trimester screening program in Denmark was extremely rigorous, resulting in detection of the vast majority of cases of Down syndrome; some cases, however, still slip through the net.
"We only have a detection rate of around 90%-92%, so we inform women that we will not be able to detect all of the Down syndrome fetuses," said Dr. Tabor, professor of fetal medicine at Copenhagen University Hospital. "This would offer a way to pick up some of the ones we haven’t found the first time."
The overall incidence of short femur length in the cohort was 2,718 cases (1.8%).
Dr. Tabor said that short femur length was a well-known feature of Down syndrome, although she was surprised by the strength of the association between short femur length and unbalanced autosomal structural abnormality.
"Every day in your clinical life, when you have a fetus where you measure this short femur length, you wonder ‘Should I do something about it or do I just ignore it?’ " Dr. Tabor said in an interview. "Do you want to do an amniocentesis if you don’t have a karyotype for the fetus, or should you really monitor the growth because they are more likely to be growth retarded or to be delivered preterm?"
Dr. Tabor said her decision about whether to act on a short femur length detected in the second trimester would depend a lot on the mother’s risk of abnormality in the first trimester.
"So if she had a risk estimate like 1 in 10,000, you probably wouldn’t do anything, but if it was 1 in 400, then you’d have to counsel her," she said.
There were no conflicts of interest declared.