What is the standard biparietal diameter?

Jul 01, 2025 Source: Cainiu Health

Dr. Zhang Lu

Introduction

Under normal circumstances, the standard values for the fetal biparietal diameter (BPD) vary according to gestational age. During pregnancy, the normal range for fetal biparietal diameter is approximately as follows: 20 mm at 12 weeks gestation, 34 mm at 16 weeks gestation, 59 mm at 24 weeks gestation, 70 mm at 28 weeks gestation, 79 mm at 32 weeks gestation, and 87 mm at 36 weeks gestation.

Generally, the standard value of the biparietal diameter varies with gestational age, and there is no fixed numerical value. A detailed analysis is as follows:

The biparietal diameter refers to the widest distance between the two sides of the fetal head and is one of the important indicators used during pregnancy to assess fetal growth and development. During pregnancy, the normal range of fetal biparietal diameter is approximately: 20mm at 12 weeks gestation, 34mm at 16 weeks, 59mm at 24 weeks, 70mm at 28 weeks, 79mm at 32 weeks, 87mm at 36 weeks, and between 88-91mm from 37 to 40 weeks gestation.

Prior to 32 weeks gestation, the biparietal diameter increases at a relatively faster rate, after which the growth rate slows down. However, biparietal diameter varies among individuals, and slight deviations above or below the average are considered normal and should not be a cause for excessive concern. If the biparietal diameter exceeds 100mm, attention should be given to the possibility of fetal macrosomia, requiring further monitoring of other indicators such as shoulder diameter and chest diameter to assess the risk of delivery complications.

In daily life, pregnant women should maintain a balanced diet and ensure adequate intake of essential nutrients, particularly high-quality proteins, while avoiding excessive nutrition and managing weight appropriately. Additionally, regular prenatal screenings should be conducted to closely monitor changes in fetal growth indicators such as biparietal diameter, enabling timely detection and management of any abnormalities.

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How big is a four-month-old fetus?

Under normal circumstances, a four-month-old fetus usually has a body length of 15–16 cm, a weight of approximately 110–150 grams, a crown-rump length of about 12–13 cm, and a biparietal diameter of around 3–4 cm. Pregnant women should maintain healthy lifestyle and dietary habits during pregnancy, and undergo regular prenatal checkups to ensure the fetus's healthy development. If any discomfort occurs, prompt medical consultation is recommended.

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What are the standard biparietal diameter and femur length at 27 weeks?

At 27 weeks, the standard biparietal diameter of a fetus is approximately 6.98 cm, and the standard femur length is around 5.10 cm. These two measurements are important indicators for assessing fetal head and lower limb development. If the examination results significantly deviate from the standard values or show abnormal growth, it is recommended to seek medical advice promptly for further evaluation. The biparietal diameter reflects the developmental status of the fetal head.

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Does a normal biparietal diameter with slightly small head circumference have any impact?

In general, whether a normal biparietal diameter with a slightly small head circumference has any impact needs to be comprehensively analyzed based on specific data and gestational age. Pregnant women should maintain a balanced diet, consume adequate high-quality protein and DHA, follow a regular作息 (daily routine), avoid fatigue, stay in good spirits, and strictly adhere to medical advice for prenatal checkups and related adjustments.

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What does a fetal skull appearing as a circular strong echo ring mean?

If the hyperechoic ring is overall round, with smooth edges and no defects, clearly outlining the fetal head contour, and if associated measurements such as head circumference and biparietal diameter are consistent with gestational age, this indicates normal fetal skull development without significant structural abnormalities. This appearance represents a typical normal finding of the fetal skull on ultrasound, reflecting an intact protective bony structure around the intracranial tissues.