New formulae for calculating Lung-to-Head-Ratio in normal fetuses between 20 and 40 weeks of gestation

Purpose: In prenatal management of fetuses with congenital diaphragmatic hernia (CDH), the prediction of the likely outcome relies on indirect assessment of fetal lung volume. Herefore, the sonographic measurement of the fetal lung area to head circumference ratio (LHR) is used. The observed to expected LHR (o/e LHR) was introduced to eliminate the gestational effect. To calculate the o/e LHR the measured LHR should be divided by the expected mean LHR for gestational age. The expected mean LHR is calculated using formulae that were developed in a study with an inappropriate population. For this, the aim of this study was to develop new formulae for expected LHR in normal singleton pregnancies between 20 and 40 weeks of gestation. Since there are different methods to measure the LHR, formulae for every method (tracing method, longest diameter method and anteroposterior method) should be established.

Material and methods: LHR and complete fetal biometry of 126 fetuses between 20 and 40 weeks of gestation were prospectively measured. Hereby, LHR was measured with three different methods (longest diameter, anteroposterior diameter, and tracing). Formulae to predict right and left LHR with regard to gestational age and biometric parameters were derived by stepwise regression analysis.

Results: The new formulae for each measuring method to calculate right and left LHR only included gestational age and no biometric parameters.

Conclusion: The new formulae for the three different methods to estimate expected LHR in normal singleton pregnancies up to 40 weeks of gestation may help to improve prognostic power of observed to expected LHR assessment in fetuses with CDH.