What Does BPD Mean in Ultrasound?
BPD, short for biparietal diameter, is a key fetal biometric measurement taken during an ultrasound scan. It refers to the distance between the two parietal bones of the fetal skull; measured from one side of the head to the other at its widest point. This value is crucial in determining gestational age, assessing fetal growth, and evaluating overall development during pregnancy.
BPD ultrasound is typically performed during the second trimester, often around the 18–22-week anatomy scan, although it can be used earlier or later depending on the clinical scenario. The measurement is performed using a cross-sectional image of the fetal head, ideally with the fetus in a neutral position. The biparietal diameter is one of several standard fetal growth measurements, alongside head circumference (HC), abdominal circumference (AC), and femur length (FL).
With the advancement of portable and point-of-care technology like Sono Mobile™C6, fetal biometric measurements such as BPD can now be obtained in more settings; including clinics, remote locations, or even during home visits by trained professionals. This increases accessibility to quality prenatal care and early detection of growth abnormalities.
Why Is BPD Important in Fetal Growth?
The biparietal diameter is not just a number on a report; it is a powerful tool for clinicians to monitor the progression of a healthy pregnancy. Here are the main reasons why BPD ultrasound matters:
Gestational Age Estimation
In early to mid-pregnancy, BPD is highly correlated with gestational age. When combined with other biometric parameters, BPD helps estimate how far along a pregnancy is; especially when the exact date of the last menstrual period is uncertain.
Fetal Growth Monitoring
Tracking the BPD across multiple ultrasound exams enables obstetricians to monitor growth velocity. If the BPD increases appropriately over time, it’s a good sign of normal development. If it plateaus or grows too rapidly, it may indicate intrauterine growth restriction (IUGR) or macrosomia.
Assessing Fetal Brain Development
Because BPD reflects skull growth, it indirectly provides insight into fetal brain development. A significantly small BPD may raise concerns about conditions like microcephaly, whereas an abnormally large BPD may indicate macrocephaly or other anomalies.
Planning for Delivery
Later in pregnancy, BPD helps estimate fetal weight, which is critical in anticipating delivery complications. For example, a very large BPD might suggest a higher risk for cephalopelvic disproportion (CPD), possibly affecting decisions around induction or cesarean section.
Normal BPD Values by Gestational Week
Understanding what’s “normal” is a key concern for expectant parents reviewing their ultrasound reports. The following chart outlines the typical range of biparietal diameter values (in millimeters) by gestational week. These numbers are approximate and may vary slightly by population, ultrasound equipment, or reference standard.
Gestational Age (Weeks) | Normal BPD Range (mm) |
13 weeks | 22 – 25 mm |
14 weeks | 25 – 28 mm |
15 weeks | 28 – 31 mm |
16 weeks | 31 – 34 mm |
17 weeks | 34 – 37 mm |
18 weeks | 37 – 41 mm |
19 weeks | 41 – 45 mm |
20 weeks | 45 – 49 mm |
21 weeks | 49 – 53 mm |
22 weeks | 53 – 56 mm |
23 weeks | 56 – 60 mm |
24 weeks | 60 – 63 mm |
25 weeks | 63 – 66 mm |
26 weeks | 66 – 69 mm |
27 weeks | 69 – 72 mm |
28 weeks | 72 – 75 mm |
29 weeks | 75 – 78 mm |
30 weeks | 78 – 81 mm |
31 weeks | 81 – 83 mm |
32 weeks | 83 – 86 mm |
33 weeks | 86 – 88 mm |
34 weeks | 88 – 91 mm |
35 weeks | 91 – 93 mm |
36 weeks | 93 – 95 mm |
37 weeks | 95 – 97 mm |
38 weeks | 97 – 99 mm |
39 weeks | 99 – 100 mm |
40 weeks | 100 – 102 mm |
Note: Always interpret these values in context — your healthcare provider will evaluate multiple parameters and consider your baby’s growth trend over time.
How BPD Is Measured
The accuracy of the BPD ultrasound depends heavily on correct technique and the position of the fetus. Here’s how sonographers measure the biparietal diameter:
Proper Plane of Measurement
The ideal view for BPD measurement is a transverse section of the fetal head; at the level of the thalami and the cavum septi pellucidi. The falx cerebri (midline structure) should be centered, and the image should include symmetric hemispheres.
Caliper Placement
The calipers are placed from the outer edge of the near parietal bone to the inner edge of the far parietal bone. This method is known as “outer-to-inner” measurement and ensures consistency across scans.
Patient Preparation
There is generally no special preparation required. However, a partially filled bladder may improve visibility in early pregnancy. Use of a high-resolution portable device, such as Sono Mobile™C6, allows for detailed and reliable BPD measurements even in outpatient or mobile clinical settings.
Challenges in Measurement
- Fetal position: If the head is tilted or the fetus is moving a lot, the technician may have to wait or ask the patient to shift.
- Skull abnormalities: Conditions such as ventriculomegaly or dolichocephaly can affect measurement accuracy.
- Operator skill: Accurate measurement requires well-trained sonographers who are familiar with fetal anatomy and biometry.
FAQs: Is a Low or High BPD Concerning?
What if my baby’s BPD is smaller than expected?
A small biparietal diameter may indicate:
- Wrong dating (i.e., your gestational age is off)
- Intrauterine growth restriction (IUGR)
- Microcephaly or brain development issues
However, one low BPD doesn’t automatically mean there’s a problem. Your provider will often recommend serial ultrasounds to monitor growth trends over time.
What if the BPD is too large?
An abnormally high BPD may suggest:
- Incorrect gestational dating
- Macrosomia (larger-than-average baby)
- Hydrocephalus or other congenital issues
Again, context matters. If all other measurements are proportionally large, it might just be a big healthy baby.
Can BPD be used alone to diagnose anything?
No; BPD is part of a comprehensive fetal growth assessment. It is used in combination with head circumference, abdominal circumference, and femur length to create a fuller picture of fetal health.
Does fetal position affect BPD accuracy?
Yes. If the fetal head is oblique or compressed, BPD might be underestimated or distorted. Skilled sonographers and portable technology like Sono Mobile™ help mitigate these challenges.
Conclusion: Interpreting BPD with Confidence
Understanding BPD ultrasound measurements empowers both expectant parents and students of medicine. Whether you’re monitoring a healthy pregnancy or investigating a growth concern, biparietal diameter offers vital clues.
With advanced tools like Sono Mobile™, fetal biometry can be conducted with accuracy, portability, and accessibility; even in settings without full radiology departments. Expectant parents can receive real-time assessments in remote communities, while students can practice biometry with affordable and practical ultrasound technology.
When interpreting any ultrasound, especially BPD, always remember:
- One scan provides a snapshot; trends tell the real story.
- Small variations in BPD are often normal.
- Communication with your provider is key.
Whether you’re reading your own report or preparing for exams, understanding the meaning, measurement, and normal values of BPD helps demystify an important part of fetal care.
Key Takeaways
- BPD (biparietal diameter) measures the width of the fetal skull and is a core part of fetal growth assessment.
- It is typically measured during the second trimester, but can be useful throughout pregnancy.
- Normal BPD ranges vary by gestational week and should be interpreted in context.
- Tools like Sono Mobile™ C6 make fetal biometry more accessible and affordable than ever.
- Speak to your healthcare provider if you have concerns about your ultrasound measurements.
✅ Want to learn more about fetal ultrasound and Sono Mobile™ solutions for home care, clinics, and remote areas? Visit www.fagonex.com or contact us directly for demos, training, and professional consultation.