Understanding BPD: Fetal Head Growth and Gestational Age Explained

Biparietal diameter refers to the distance between the carina of the two parietals of the fetal head, located about the length between the bones at their widest points above the temples on both sides.

As the gestational age increases, the biparietal diameter gradually increases, which is used to estimate the gestational age and monitor the size of fetal development.

Biparietal diameter at different gestational weeks

What’s the normal range?

The range of biparietal diameters at different gestational weeks is detailed in the table below, where BPD is biparietal diameter and BPDc is corrected biparietal diameter.

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In clinical practice, in addition to biparietal diameter, head circumference is also an important reference indicator. When we encounter a biparietal diameter that is too large or too small, we will refer to the head circumference indicator at the same time to see if further evaluation is needed. Next Let’s talk about it in detail.

Is there a problem with the large biparietal diameter of the fetus?

The biparietal diameter is large and there is no abnormality in the brain

If the fetal biparietal diameter is too large, but there is no abnormality in the fetal cranial structure, it is generally suggested that there is the possibility of a macrosomia.

This condition may be related to genetic factors, but babies with excessive biparietal diameter are more likely to have dystocia during normal delivery, and expectant mothers need to choose the appropriate delivery method based on the doctor’s judgment.

Because the baby’s cranial sutures are not closed, under the squeeze of the birth canal, the cranial bones overlap and the biparietal diameter shrinks.

So, some babies have a “pointy” head shape when they are first born, but they will return to their normal head shape in a few days.

The biparietal diameter is large and an anomaly is suspected

If the fetal biparietal diameter is too large and the doctor suspects an abnormality, the pregnant woman will generally be advised to undergo color ultrasound to rule out hydrocephalus. If necessary, the fetal spine, etc. will also be examined for integrity, and the problem will be judged and solved.

Note: Sometimes, different equipment and different doctors will produce different degree of measurement error, even if it is found that the biparietal diameter is too large, expectant mothers do not need to worry too much, just follow the detailed guidance of the doctor.

Is there a problem with the small biparietal diameter of the fetus?

For the standard values of the fetal biparietal diameter at different gestational weeks during pregnancy, the determination must be made by a professional doctor. If the doctor suspects that the fetal baby has less than the normal biparietal diameter, it can be considered an abnormal condition and needs to be alerted.

The biparietal diameter is significantly smaller

If the fetal biparietal diameter is significantly smaller, the fetal body weight needs to be comprehensively estimated by further combining the fetal head circumference, abdominal circumference, and femur length.

If the estimated weight of the fetus is similarly small, a physician’s determination is required as to whether there is a developmental limitation of the fetus.

In this case, the doctor needs to decide on the treatment plan based on the gestational age, amniotic fluid volume, umbilical artery S/D and intrauterine condition of the fetus.

Simple biparietal diameter and small head circumference

If the simple biparietal diameter and head circumference are too small, the development of the fetus can continue to be monitored dynamically. Generally, the observation interval is at least 2 weeks to review the growth of the biparietal diameter and head circumference.

If the growth trend is normal, then normal is highly likely;

If growth is stagnant, then a combination of chromosomal screening indicators (NT, serological screening, or noninvasive DNA) in early to mid-pregnancy is required, while, at the same time, intracranial development is assessed by fetal magnetic resonance imaging (MR I) examination, excluding microcephaly, and assessing the risk of chromosomal abnormalities.

Disclaimer: All photos used in this blog are generated by artificial intelligence (AI). These images are original creations produced by AI technology and do not depict real people, places, or events. They are provided for illustrative purposes only and cannot be claimed or used as real photographs.