BPD in the Maternity Record: Significance & Values for Expecting Mothers
What does the abbreviation BPD mean in the maternity record?
BPD stands for biparietal diameter. This is the transverse diameter of the head, measured from ear to ear of the unborn baby. The longitudinal diameter – measured from forehead to back of the head – is called the frontooccipital diameter and is abbreviated as FOD in the maternity record. The abbreviation for head circumference is KU in the maternity record. Reading tip: The maternity record contains much more; we have explained it page by page – from blood type to Rh factor to antibodies.
When and how is the biparietal diameter measured?
In an uncomplicated pregnancy, three major ultrasound examinations are planned. The fetal head is usually measured during the second (19th to 22nd week of gestation) and third screenings (29th to 32nd week of gestation) using sonography. The doctor marks the widest part of the fetus's head on both sides, and the ultrasound device then calculates the head diameter. Reading tip: Ultrasound in pregnancy – learn more about the individual examinations.
Where is the BPD recorded in the maternity record?
The BPD determined during the ultrasound is noted on the right side of page 11 of the maternity record (on page 27 for the second pregnancy) in the corresponding box under the BPD abbreviation. Usually, the head diameter values are recorded in millimeters, although sometimes the doctor may also use centimeters.
On the double page titled “Ultrasound Examinations,” you will find not only the BPD but also all other findings from the major screenings. This includes information about the position of your placenta, the humerus length (length of the upper arm bone), the femur length (length of the thigh bone), the abdominal circumference, and the organ development of your child. Rest assured: If the doctor marks “normal development” on page 10, your baby has grown appropriately for the gestational week, has developed well, and no malformations can currently be detected.
Why is the BPD determined during pregnancy?
“Normal” growth during pregnancy is the most reliable indicator that your child is healthy and well-nourished in the womb. If a fetus is noticeably small for its gestational age, it may indicate intrauterine growth restriction – that is, delayed growth. This can occur if the unborn child is not optimally supplied in the womb due to placental insufficiency or if preeclampsia is present. To detect such complications early in pregnancy and to address any abnormalities, timely development is closely monitored during ultrasound examinations. In addition to the head, the abdomen and body length of your child are also examined, but the BPD is considered one of the most important values: It is the most reliable measure for detecting delayed growth and the resulting risk of preterm birth. At the same time, the BPD, along with the FOD and head circumference, explicitly provides information about head growth. Therefore, this value is also considered individually and used to detect malformations such as hydrocephalus (commonly known as “water on the brain”) or developmental disorders like microcephaly (very small head, indicating a disruption in brain growth) at an early stage. A noticeably small BPD may also indicate that the unborn child has been infected with the cytomegalovirus. Additionally, the biparietal diameter is the most reliable way to determine how far along you are in your pregnancy during the second and third trimesters. Reading tip: Even after birth, the baby's head circumference is an indicator of healthy growth – learn more about it here.
What BPD values are considered normal?
On page 13 of the maternity record, you will find a norm curve that shows fetal growth patterns during pregnancy. Alongside the BPD, there is also the growth curve for the abdominal transverse diameter (ATD) and the crown-rump length (SSL). It is important to know: These curves are percentile curves. Percentiles are frequency measures that indicate how a particular value compares to the values of a large group. Specifically, in the case of the BPD: how large or small your baby's head is compared to the heads of other babies of the same age. The middle, thicker line represents the 50th percentile – this indicates the average value. This means that half of all children fall above this value and the other half below it, while the average value occurs slightly more frequently than the others. The 5th and 95th percentiles mark the upper and lower limits of the normal distribution. This means: Even if a fetus has a smaller BPD, it can still be completely normal. We have compiled the reference values in a table for you; these values* are also the basis for the BPD curve in the maternity record.
Table of BPD Values according to Merz & Wellek
| Gestational Week (Gestational Age) | BPD 5th Percentile | BPD 50th Percentile | BPD 95th Percentile |
|---|---|---|---|
| 12 | 12 | 25 | 29 |
| 13 | 15 | 28 | 32 |
| 26 | 36 | 61 | 63 |
| 30 | 46 | 61 | 70 |
| 35 | 39 | 73 | 81 |
| 40 | 46 | 94 | 100 |
*Data according to Merz & Wellek 1996, values in millimeters (mm)
Reading tip: If you want to know more about the curves: In our percentile article, you will find examples that explain the principle in more detail.
Small or large BPD value – when is it problematic?
As already explained, a lower value is not automatically a warning signal. If your child is moving along “its” percentile, there is usually no cause for concern. However, if the value suddenly deviates significantly upwards or downwards, the doctor will take a closer look. This also applies if the value is above the 90th or below the 10th percentile. Additionally, it can happen that an incorrect BPD is measured. For babies in breech position – that is, with their bottom down – or in transverse position in the womb, the biparietal diameter is often measured too small. Also, too little amniotic fluid (oligohydramnios) or a twin pregnancy can complicate accurate measurement due to the position of the baby. Boys also have a significantly larger BPD than girls – so a value that initially appears large may normalize after the sex is identified. However, the assumption that larger fluctuations in BPD are expected depending on ethnicity could not be proven in a large-scale international study. So: The table above can serve as a guideline but should not cause you anxiety. During the ultrasound examinations, the doctor will pay close attention and will inform you if any abnormalities arise.