Biparietal Diameter Calculator

Weeks: Biparietal Diameter (mm):
WeeksBiparietal Diameter (mm)

Reference: Snijders & Nicolaides, Ultrasound Obstet Gynecol 1994;4:34–48

What does a fetal ultrasound show?

Fetal ultrasound measurements can show how the baby is growing and detect abnormalities. During pregnancy, many different ultrasounds measurements can be done. Fetal ultrasound measurements can include the crown-rump length (CRL), biparietal diameter (BPD), femur length (FL), head circumference (HC), occipitofrontal diameter (OFD), abdominal circumference (AC), and humerus length (HL), as well as calculation of the estimated fetal weight (EFW). 

Why is a biparietal diameter measurement important?

The biparietal diameter (BPD) is one of many measurements that are taken during an initial pregnancy ultrasound. The BPD is a measurement of the diameter across your developing baby's skull, from one parietal bone to the other. Doctors use the BPD to determine gestational age and also to estimate fetal weight.

In addition to the BPD, the typical measurements taken are the crown-rump length (CRL), abdominal circumference (AB), femur length (FL), and head circumference (HC).

Fetal Measurements and Due Date

One of the most important reasons for tracking fetal measurements is to pinpoint a due date. There are several guidelines to calculate the estimated due date using fetal measurements:

  1. If the last menstrual period (LMP) is unclear then the crown-rump length before 14 weeks can be used to calculate the estimated due date
  2. If the CRL before 9 weeks is within 5 days of the estimated due date calculated by the first day of the last menstrual period (LMP) then LMP should be used to calculate the estimated due date. 
  3. If the CRL before 9 weeks is more or less than 5 days different from the first day of the last menstrual period (LMP) then the ultrasound date of the estimated due date should be used. 
  4. If the CRL between 9 weeks and 13 weeks is more or less than 7 days different from the date calculated from the first day of the last menstrual period (LMP) then the ultrasound date of the estimated due date should be used. 
  5. If the biparietal diameter/head circumference/abdominal circumference/femur length between 14 weeks-15 weeks is more or less than 7 days different from the date calculated from the first day of the last menstrual period (LMP) then the ultrasound date of the estimated due date should be used.
  6. If the biparietal diameter/head circumference/abdominal circumference/femur length between 16 weeks-21 weeks is more or less than 10 days different from the date calculated from the first day of the last menstrual period then the ultrasound date of the estimated due date should be used. 
  7. If the biparietal diameter/head circumference/abdominal circumference/femur length between 22 weeks-27 weeks is more or less than 14 days different from the date calculated from the first day of the last menstrual period (LMP) then the ultrasound date of the estimated due date should be used. 
  8. If the biparietal diameter/head circumference/abdominal circumference/femur length between 28 weeks and beyond is more or less than 21 days different from the date calculated from the first day of the last menstrual period then the ultrasound date of the estimated due date should be used. 

Read More:
Ultrasound Guide