What is biparietal size?

Biparietal size is one of the indicators of fetal development. How is it determined? What do the deviations indicate?

biparietal size

Biparietal fetal head size

This index is among those that can determine the duration of pregnancy and the presence of deviations in fetal development. This indicator gives the most accurate data on the duration of gestation. The biparietal size is determined by ultrasound. The data are most informative in the period from the twelfth to the twenty-eighth week of pregnancy.

Biparietal head size: normal

biparietal head size

BPR is the distance from the outer to the inner contour of the parietal bones. The line that connects the outer contours should be located above the thalamus. If the measurement is incorrect, incorrect results will be obtained. So, the period of gestation will be determined erroneously. For each gestational age, the normal BDP value is determined. With each week, the rate increases. In the third trimester, growth becomes slower.For 12 weeks, the indicator is 21 mm, in 13 - 24 mm, in 16 - 34 mm, in 24 - 61 mm, in 32 - 82 mm, in 38 - 84 mm, in 40 - 96 mm. When evaluating the BDP, the LZR indicators (frontal-occipital size) are taken into account. The measurement is performed in the same plane (at the level of the visual mounds and the legs of the brain). Both indicators vary according to gestational age. Indicator BPR may vary depending on the configuration of the head, which changes after 38 weeks. If the dolichocephalic configuration occurs, the biparietal size will be smaller than the norm.

What pathologies can be identified by BPD?

In accordance with the value of BPD and other indicators, it is possible to determine the presence of intrauterine growth retardation, large fetus and hydrocephalus. In some cases, the biparietal size may exceed the norm, but this may not indicate the presence of deviations. It is important to measure the rest of the body.

  • If the indices of the abdomen, chest and head are increased evenly, it can be assumed that the fetus is large.
  • If only BPR and LZR exceed the norm, hydrocephalus may be present. The cause of this disease is intrauterine infection.biparietal head size
  • In cases where the biparietal size is below normal and the other indicators do not correspond to the gestational "age", it is possible to speak of developmental delay. Causes include chronic hypoxia, placental insufficiency, intrauterine infections. In this case, treatment must be carried out, which is aimed at eliminating the causes of IUGR - increasing the delivery of nutrients and oxygen to the fetus (Pentoxifyline, Actovegin, Curantil) and improvement of the uteroplacental circulation.
  • If the body size of the fetus corresponds to the period of gestation, and LZR and BPR is less than the norm, it shows the presence of microcephaly.


With correct and timely detection of abnormalities, it is important to start treatment on time. In many cases, you can change something. But based on data from one study is not worth it. If the figures are significantly different from the norm, it is better to re-ultrasound.

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