What is BPD in Pregnancy?
Summary
- Definition: In pregnancy, BPD stands for Biparietal Diameter. It is the measurement of the distance between the two sides of your baby’s head (from one parietal bone to the other).
- Purpose: Doctors use this measurement during ultrasound scans to estimate your baby’s gestational age, track brain development, and estimate foetal weight.
- Timing: BPD is typically measured from the late first trimester (around 13 weeks) until the end of your pregnancy.
- Normal ranges: A normal BPD increases as your pregnancy progresses. If your baby’s BPD is slightly smaller or larger than average, it is often due to genetics rather than a medical concern.
- Next Steps: Always discuss your scan reports with your obstetrician, who will look at the BPD alongside other measurements like head circumference (HC) and femur length (FL) for a complete picture of your baby’s health.
Hello! I am Dr. Supriya Puranik. If you are reading this, chances are you have just returned from your anomaly scan or growth ultrasound, and you are staring at a piece of paper filled with medical abbreviations.
Pregnancy is a beautiful journey, but looking at medical reports can sometimes feel like trying to read a foreign language.
One of the most common abbreviations you will see on your ultrasound report is “BPD”.
When parents see a number next to this, they often feel a rush of anxiety, wondering what it means for their growing baby.
Let us sit down together, take a deep breath, and decode this medical jargon.
I want you to feel confident and informed about what is happening inside your womb.
What exactly does BPD stand for in my ultrasound report?
In the context of obstetrics and prenatal scans, BPD stands for Biparietal Diameter.
Simply put, it is the measurement of the diameter of your baby’s growing head.
When the sonographer moves the ultrasound wand over your belly, they look for a specific cross-section of the baby’s skull.
They then measure the distance from one parietal bone (the side of the skull) to the opposite parietal bone.
Imagine measuring from just above one ear to just above the other. That distance is the biparietal diameter.
It is one of the four standard measurements we use to check on your baby, alongside Head Circumference (HC), Abdominal Circumference (AC), and Femur Length (FL).
(Note: In a psychiatric context, BPD can stand for Borderline Personality Disorder. However, if you see this term on a pregnancy ultrasound report, it strictly refers to the measurement of your baby’s head.)
Why do we measure your baby’s BPD?
Measuring your baby’s head gives us incredible insight into how they are developing. We do not just measure it for the sake of adding numbers to a chart. We use the Biparietal Diameter for a few vital reasons:
- To confirm your gestational age and due date
During your earlier scans, the BPD is highly accurate in telling us exactly how many weeks pregnant you are. If you are unsure of the date of your last menstrual period, this measurement helps us pinpoint your estimated date of delivery. - To estimate your baby’s weight
We cannot put a baby on a scale while they are still inside the womb. Instead, we use a combination of BPD, abdominal circumference, and femur length to calculate an estimated foetal weight. This helps us ensure your baby is growing at a healthy rate. - To monitor healthy brain development
The skull grows to accommodate the developing brain. By tracking the BPD, we can ensure your baby’s head—and therefore their brain—is growing exactly as it should be.
When do we usually measure the biparietal diameter?
We cannot measure the BPD right at the beginning of your pregnancy.
In the very early weeks, your baby is simply too small, and we use a different measurement called the Crown-Rump Length (CRL) instead.
We typically start measuring the Biparietal Diameter towards the end of your first trimester, usually around 12 to 13 weeks of gestation.
From that point onward, we will measure it during every routine growth scan until you give birth.
It is particularly important during your 20-week anomaly scan, where we look closely at your baby’s anatomy.
What is a normal BPD measurement?
This is the most common question expecting mothers ask me in my clinic. The truth is, “normal” is a moving target.
At 13 weeks, your baby’s BPD might be just around 2.4 centimetres. By the time you reach 36 weeks, it will have grown to about 9 centimetres.
Instead of focusing on a single number, we look at where your baby’s measurement falls on a percentile chart.
If your baby’s BPD falls between the 10th and 90th percentile for your current week of pregnancy, we consider it completely normal.
What if your baby’s BPD is outside the normal range?
It is incredibly easy to panic if your scan shows a BPD that is larger or smaller than expected. Please, do not worry prematurely.
If the BPD is larger than average, it simply means your baby has a slightly larger head. Look at your partner, and look at your own family.
Do people in your family naturally have larger heads? Genetics play a massive role here.
In rare cases, a significantly large BPD might prompt us to look for conditions like gestational diabetes or excess fluid, but we will always guide you through any extra checks if necessary.
If the BPD is smaller than average, it could also just be genetic. Sometimes, it means the baby’s head is shaped slightly differently—perhaps a bit longer and narrower.
We will always look at the Head Circumference (HC) alongside the BPD. If the HC is normal, a small BPD is usually nothing to worry about.
If the baby is measuring small overall, we will monitor your placenta function and fluid levels to ensure your baby is getting all the nutrients they need.
Can a small or large BPD affect your delivery?
As you approach your third trimester, you might wonder if a large BPD means you will have a difficult labour.
While a larger head can sometimes make a vaginal delivery slightly more challenging, the female body is incredibly well-designed for childbirth.
Your pelvis naturally opens, and the baby’s skull bones are not fully fused yet.
They are designed to overlap slightly (a process called moulding) to help your baby navigate the birth canal safely.
During your final weeks of pregnancy, we will assess the size of the baby and discuss the best, safest delivery plan for you.
How can we help you through your pregnancy journey?
Understanding your ultrasound report is just one small part of your pregnancy.
What truly matters is having an experienced, compassionate medical team by your side to interpret these results and provide personalised care.
I believe that no mother should ever leave a scan room feeling confused or anxious.
You deserve transparent conversations, expert medical care, and a doctor who listens to your concerns.
If you are looking for comprehensive maternity care, or if you need a second opinion on your recent ultrasound reports, my team and I are here for you.
We specialise in both routine and high-risk pregnancies, ensuring that you and your baby receive the highest standard of monitoring and support.
Let us turn your pregnancy worries into confidence.
Book a consultation with us today, bring your reports, and let us take the next step of this beautiful journey together.
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About Author
Dr. Supriya Puranik
Gynaecologist & IVF Specialist
MMC -072514 (1993)
Dr. Supriya Puranik, a renowned gynaecologist and infertility expert, leads the IVF & Gynaecology department at Sahyadri Hospitals Momstory in Shivaji Nagar, Pune. She is committed to helping couples overcome infertility challenges.
