Q&a attempts to provide balanced answers to those curly-yet-common questions in obstetrics and gynaecology for the broader O&G Magazine readership, including Diplomates, Trainees, medical students and other health professionals.
‘I have a patient whose routine morphology ultrasound, performed at 19 weeks, revealed normal morphologic appearances, placental position and liquor
volume, but measurements suggesting the fetus was symmetrically small, about two weeks smaller than the dates. The dates are very certain and she had an ultrasound at 12 weeks (with a low-risk NTS result). How should I handle this and what should my plan be?
a The unexpectedly small fetus is a puzzling problem and can cause great distress to women. Of course, the commonest cause of a fetus that is morphologically normal, but symmetrically small is simply wrong dates. In this case, the dates appear to be accurate, but it is always worth reviewing the earlier imaging to make sure that measurements from the nuchal translucency scan were accurately taken. It is also useful to re-check the PAPP-A result, if it was done, to see whether it was low and might be associated with very early-onset growth restriction, and to review the maternal medical history.
Assuming that the dates are accurate and that the nuchal translucency screen yielded a low-risk result, then an assessment of the parents is the next step. It may seem obvious, but a very petite mother and small father can have a small baby. If the liquor volume is normal and the Doppler waveform of the uterine arteries is normal, it is worth checking for congenital infection. This includes the ‘TORCH’ serology – toxoplasmosis, rubella, cytomegalovirus and herpes.
It is definitely worth having a second pair of eyes check the 20-week scan, (in particular the heart and outflow tracts), by having a second opinion from another skilled and experienced imaging specialist, such as a maternal-fetal medicine subspecialist or obstetric ultrasound subspecialist. At the time, an amniocentesis or non-invasive prenatal testing (NIPT) could be discussed, particularly if there are any soft signs or suspicious anatomic features elicited – an appropriate quantitative PCR can also be undertaken for congenital infection on the specimen of liquor.
Whatever the outcome of these investigations, serial ultrasound is important. In this case, I would repeat the ultrasound in two weeks to assess growth velocity, dopplers (umbilical artery, ductus venosus and middle cerebral artery), amniotic fluid volume and to see if any structural abnormalities with the fetus become more evident (such as a reduction in limb growth velocity, suggesting skeletal dysplasia). Timing of ultrasound assessment afterwards depend on the hypothesised cause, as appropriately timed premature delivery as prompted by fetal surveillance with ultrasound (and CTGs after 26 weeks) may reduce the risk of stillbirth in an otherwise normal baby with placental insufficiency.