Inpatient Obstetric Nurse Practice Exam 2025 – Complete Prep Guide

The biparietal diameter (BPD) is a measurement of the fetal head used primarily in ultrasound to assess fetal growth and estimate gestational age. It is most accurate for estimating gestational age during the second trimester.

In the first trimester, measurements such as crown-rump length (CRL) are more precise for dating because the embryonic structures are rapidly developing and have a more consistent morphology. As pregnancy progresses into the second trimester, fetal structures, including the skull, are better formed, allowing for more reliable measurements from BPD.

Moreover, BPD can become less reliable in the third trimester due to variations in fetal head shape and size, which can be influenced by factors such as genetics, fetal position, and maternal health. These variations can lead to discrepancies in estimating gestational age using BPD in later stages.

Therefore, using BPD for estimating gestational age is best suited for the second trimester, when the measurement provides a reliable indication of growth and development.