Perinatal outcomes following mid trimester detection of isolated short foetal femur length

Abstract

Background: Short foetal femur length (FL) is a normal variant but may also be a marker for disorders such as skeletal dysplasia, Trisomy 21 (T21), Turners syndrome, congenital infection (TORCH) and foetal growth restriction (FGR).Aim: Our aim was to review outcomes in our population following a diagnosis of isolated short FL (FL <5th centile) when detected at the time of mid trimester foetal anomaly scan (FAS).Methods: All women within NHS Tayside who attended for routine mid trimester foetal anomaly scan at 18–21 weeks gestation (Range = 18+2–21+3 weeks) between November 2011 and June 2016 were included. Those who had an isolated FL <5th centile were identified using Viewpoint. Data relating to perinatal and childhood outcomes were obtained from local databases.Results: 72 women were identified. The median maternal age was 30 years (range = 17–45 years). 39/72 (54.2%) women were primigravid. 41/72 (56.9%) women had Down’s syndrome screening (DSS). 12/72 (16.7%) women were offered TORCH screens, none of which were positive for recent or current infection.Invasive testing was performed in 8/72 (11%) women; 1 had previously had first trimester chorionic villus sampling (CVS) for raised maternal age. 7 women had amniocentesis (1 prior to FAS for increased DSS risk). All those that had invasive testing had normal foetal karyotype.Median gestational age at delivery was 38 weeks (range = 26–41 weeks). 10/72 (13.9%) had genetic testing after birth, 6 were performed shortly after birth due to dysmorphic features. 4/72 (5.5%) had Trisomy 21 and 3 of these had VSD that had not been identified antenatally. For those with T21, 3 mothers had declined DSS and 1 had a low risk DSS result.Fifty out of seventy two (69.4%) babies had constitutionally short femurs. 13/72 (18%) had FGR and 3/72 (4.2%) had skeletal dysplasia. One baby had William’s syndrome, one had Klinefelter syndrome and two had genetic deletions of uncertain significance.Conclusions: Knowledge of outcomes will enable better counselling for women with isolated short foetal femur. In our local population, more than two-thirds of the babies with isolated short femur will be normal. Women must, however, be informed of the potential for foetal aneuploidy and foetal skeletal dysplasia. In view of this, women should be offered invasive testing and follow-up ultrasound scans to assess the foetal skeleton. Furthermore isolated short foetal femur is a marker for FGR and all women with isolated short femur should be offered serial ultrasound scans to assess foetal growth and well-being in the third trimester.