Normal biparietal diameter (BPD) at 38 weeks of gestation

Using biparietal diameter (BPD) to assess fetal size provides only a reference value. So, what is the normal BPD at 38 weeks of gestation?

Normal Biparietal Diameter at 38 Weeks of Gestation

At 38 weeks of pregnancy, the normal biparietal diameter (BPD) of the fetus typically falls within the range of 9.08 ± 0.59 cm. The BPD refers to the maximum transverse diameter of the fetal head—i.e., the distance between the two parietal bones—and is commonly termed the “major fetal head diameter.” As gestational age advances, the BPD gradually increases. However, since each fetus develops as an independent individual, growth does not follow standardized measurements precisely. Therefore, as long as the BPD remains within the normal reference range, fetal development is considered normal. Clinically, experienced obstetricians do not memorize exact numerical values; they rely instead on general assessments of normality. In fact, a BPD exceeding 8.5 cm after 38 weeks’ gestation is entirely normal.

Gestational age is not determined solely—or even primarily—by BPD or fetal size. Instead, clinicians integrate multiple biometric parameters, including abdominal circumference (AC) and femur length (FL), for comprehensive assessment. At 38 weeks, BPD changes very little; variations may reflect genetic factors, maternal nutrition, or underlying maternal conditions. Minor deviations above or below the average are generally insignificant, provided the pelvis is anatomically normal and no other complications are present.

In late pregnancy, expectant mothers should attend regular antenatal check-ups and closely monitor fetal movements. Prompt medical consultation is essential if symptoms such as vaginal bleeding or abdominal pain occur. We hope this information has been helpful. Wishing you good health and happiness!