Fetal Gestational Age Determination using Ultrasound Placental Thickness

The purposes of this study are to sonographically measure the placental thickness (PT) in normal fetuses; to correlate it with gestational age (GA), fetal growth parameters, and estimated fetal weight (EFW); and to design a nomogram for the derived PT measurements.

This was a hospital-based cross-sectional study on 400 women with apparently normal pregnancy within the age range of 18-45 years recruited from the Antenatal Clinic of our hospital. The fetal GA was estimated by the last menstrual period (LMP). The fetal growth parameters were determined using standard sonographic methods while the PT was measured at the level of the umbilical cord insertion site. PT was then correlated with GA, fetal growth parameters, and the EFW.

The mean PT (mean ± standard deviation) in the 1^st, 2^nd, 3^rd trimesters and the whole duration of pregnancy were 14.5 ± 0.3 mm, 24.6 ± 3.9 mm, 34.8 ± 2.8 mm, and 29.6 ± 7.1 mm, respectively. PT ranged from 13.5 ± 1.9 mm at 11 weeks to 39.1 ± 0.6 mm at 40 weeks. PT (in mm) had a linear relationship and a statistically significant positive correlation with GA (in weeks) in all the trimesters, with most significant correlation recorded in the 2^nd trimester (r = 0.79). There was also a statistically significant positive correlation between PT and the fetal growth parameters (biparietal diameter, head circumference, abdominal circumference, femur length and crown-rump length), and EFW. PT nomogram was developed from 11 to 40 weeks of gestation using a scatter plot with 95% confidence interval for our locality.

PT has a linear relationship with GA, fetal growth parameters, and EFW and it can be used along with other fetal growth parameters to increase the accuracy for predicting GA in normal pregnancies, especially when the subject is not sure of or does not know her LMP.