Ultrasonographic estimation of gestational age is done routinely by using fetal biometric parameters, such as the biparietal diameter (BPD), femur length (FL), abdominal circumference (AC), and head circumference (HC). Gestational dating by ultrasound is reliable in the 1st trimester, but discrepancies increase to more than 3 weeks in the 3rd trimester.
To study the accuracy of gestational age estimation by the transcerebellar diameter (TCD) in the 3rd trimester of singleton pregnancies whose gestational age is known.
A prospective study was carried out in the Department of Obstetrics and Gynecology at NDMC Medical College and Hindu Rao Hospital, Delhi, India, from October 2017 to May 2019 after obtaining clearance from the ethical committee. The study included 100 women aged 18-35 years with singleton pregnancies in the 3rd trimester with a gestational age ≥ 28 weeks; the women provided informed consent and filled in an F-form to rule out any sex determination of the baby. Only those women who had regular menstrual cycles 6 months before conception, who were sure of their last menstrual period (LMP), who had a gestational dating scan done up to 14 weeks, and for whom congenital malformation of the fetus had been ruled out by mid-trimester ultrasound were included. Gestational age was obtained for the measured BPD, HC, AC, FL, and TCD. A paired t-test was used for the comparison between the period of gestation (POG) by the 1st ultrasonography (USG) and that by the AC, BPD, FL, HC, and TCD.
The estimation of gestational age by the BPD becomes gradually less reliable after 32 weeks of pregnancy, that by the HC and FL after 36 weeks, and that by the AC after 28 weeks. Regression analysis was used to find a correlation of the various parameters, such as the POG by the 1st USG, BPD, HC, AC, FL, and TCD, with the POG by LMP. The correlation coefficient (r), coefficient of determination (R), and p values were calculated. Of all the sonographic parameters for the estimation of gestational age, the TCD showed the highest correlation (r = 0.979; p < 0.0001) and the AC the lowest correlation.
The mean difference between the estimate of gestational age by the TCD and that by the 1st trimester USG increased from ± 1 day at weeks 28-32 to ± 1-2 days at weeks 32-36 and further increased to ± 6 days after 36 weeks of pregnancy, which was statistically significant. This indicated that the TCD could estimate gestational age within 6 days of near-term pregnancy. Thus, the TCD was a reliable parameter for the estimation of gestational age within 6 days of near-term pregnancy along with routine biometry and an alternative parameter for the evaluation of gestational age when the LMP is not known.

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