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BPD And FL Chart in Pregnancy – Biparietal Diameter and Femur Length in Ultrasound


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Pregnancy term and ultrasound imaging go hand in hand. Not only do you get to witness your baby’s growth, but the ultrasound report also ascertains the health of the fetus, their gestational period, and even their development in the mother’s womb.

After an ultrasound, your OBGYN will likely discuss “BPD and FL chart in pregnancy.” This stands for Biparietal diameter (diameter of the baby’s developing skull bone and structure) and the Femur length (length of the baby’s growing and developing femur).

Through ultrasound imaging, these factors can help ascertain the overall fetal weight and the baby’s gestational age. Comparing the numbers of BPD and FL with the standard chart also allows the doctors to look out for underdevelopment, overdevelopment and other developmental anomalies in the fetus.

If this is your first pregnancy and you are confused about these pregnancy-related terminologies like BPD and FL, let us explain them to you in simpler words and more detail.

What is BPD, and what is its significance?

BPD or Biparietal diameter is a crucial measurement that ascertains the baby’s skull development and growth. It is the length between the two parietal bones on either side of the skull of the developing baby.

Every growing fetus has two parietal bones – present on either side of the side. It has a curved-plate shape with two surfaces. The reason why ultrasound technicians calculate or measure the BPD is to find the fetus’s gestational age. It also gives a rough idea about the baby’s weight and if it is on track with the standard healthy weight of the developing fetus.

BPD (as for FL) is calculated using digital tools while the ultrasound is ongoing.

BPD, FL Chart in Pregnancy

Weeks (mm) BPD (mm) FL
16 32.3 20.5
16.5 34.2 22.1
17 36 23.7
17.5 37.7 25.2
18 39.5 26.7
18.5 41.3 28.3
19 43 29.8
19.5 44.7 31.2
20 46.4 32.7
20.5 48.1 34.1
21 49.7 35.6
21.5 51.4 37
22 53 38.4
22.5 54.6 39.8
23 56.2 41.1
23.5 57.8 42.5
24 59.3 43.8
24.5 60.8 45.1
25 62.3 46.4
25.5 63.8 47.7
26 65.3 48.9
26.5 66.7 50.2
27 68.1 51.4
27.5 69.5 52.6
28 70.8 53.8
28.5 72.2 55
29 73.5 56.1
29.5 74.7 57.3
30 76 58.4
30.5 77.2 59.5
31 78.4 60.6
31.5 79.6 61.7
32 80.7 62.7
32.5 81.9 63.8
33 82.9 64.8
33.5 84 65.8
34 85 66.8
34.5 86 67.7
35 87 68.7
35.5 87.9 69.6
36 88.8 70.6
36.5 89.7 71.5
37 90.5 72.3
37.5 91.3 73.2
38 92.1 74.1
38.5 92.8 74.9
39 93.5 75.7
39.5 94.2 76.5
40 94.8 77.3
40.5 95.4 78.1
41 95.9 78.8
41.5 96.5 79.5
42 96.9 80.3


When is BPD measured?

Ultrasound frequency varies from one pregnant person to the other. Some people get between 1-3 ultrasound imaging up to 20 weeks. If the pregnancy is a high risk, ultrasound frequency can increase.

Besides BPD, the technician will also measure the Femur Length (FL), head circumference, and abdominal circumference of the fetus. All of these determine the baby’s healthy growth and development and check for any medical concerns.

The BPD determines the baby’s current body weight, but it also estimates if the baby’s brain is developing at a healthy rate or not. The BPD ultrasound normal range at 20 weeks is around 4.64 cm and around 9.5 centimeters by the end of the pregnancy term.

BPD is typically measured between week 12 to week 26 of the pregnancy to predict and ascertain the fetus’s gestational age. Studies even show that BPD measurement is more accurate before week 20 and can lose accuracy after week 20.


FL, BPD Values Normal Range: What is considered an abnormal BPD range?

Your OBGYN will tally it with the standard BPD-FL pregnancy chart when measuring the BPD levels. The normal range of femur length (FL) to BPD or FL/BPD ratio is calculated to be 79 +/- 8%. If the levels are outside the normal range, your doctor will prescribe further testing. There can be two significant conditions.

Lower BPD range Higher BPD range
-Sign of intrauterine growth restriction

-Flatter development of the skull

– Microcephaly

-Underlying health concern

– Possible sign of gestational diabetes

 Pregnant women who have contracted the Zika virus should get an ultrasound done every 3-4 weeks to rule out abnormalities. This is a standard recommendation by the Centers for Disease Control and Prevention (CDC).

What is Femur Length (FL) in Pregnancy?

The femur is the largest bone in the body and is a crucial determinant of the baby’s fetal growth. Much like BPD, HC, AC, FL chart measures the length of the thigh bone to ensure the standard and healthy development of the baby.

The FL is calculated using digital tools during the ultrasound imaging. The technician measures the length from the blunt end of the thigh bone to the shaft. Unlike BPD, FL is measured in millimeters.

Any issues or diversions from the standard length of the femur indicate an underlying concern that requires further testing. Although FL measurement helps determine fetal growth and length development, it comes with inaccuracies.

What happens with Short Femur Length?

The fetus has a short femur length below the fifth percentile. This requires further testing to rule out any health concerns, including the following:

Dwarfism – one of the crucial issues with short femur length is skeletal dysplasia or dwarfism. There are over 200 different categories of disorders under this condition. The majority (if not all) of them are genetic concerns. The conditions are characterized by cartilage and bone growth deformities in the fetus.

Placental insufficiency – studies suggest that a fetus has short femur length if they aren’t getting adequate nutritional requirements from the placenta during the developmental phase. This can lead to premature birth, smaller stature of the baby corresponding to their gestational age, etc.

Aneuploidies – prevalence of short femur length is also a sign of chromosomal abnormalities in the fetus. This can lead to Down syndrome, Patau syndrome, Edward’s syndrome, etc.

Any abnormalities in the femur length are a sign of concern that needs further prenatal testing. Your OBGYN will guide you accordingly after determining the concern.

What is the normal BPD-FL ratio in Ultrasound scan report?

The normal BPD-FL ratio in Ultrasound is 79 +/- 8%. This is a determinant of the fetus’ health growth and development.

Understanding these numbers and chart readings is confusing. However, your ultrasound technician and OBGYN will explain your fetus’ growth and development corresponding to the BPD-FL pregnancy chart. If there are any concerns, you will get to know about them.


BPD and FL are two of the most crucial factors determining the baby’s growth in the womb. These are determinants of the “on-track” development of the baby during the pregnancy term. Although it is often difficult and frightening to hear about possible concerns after the tests, they also rule out issues with the baby’s development.

Knowing things prior is always better than regretting things once the baby is born. Even with the marker of concern in some pregnancies, the outcome can be a healthy and full-term pregnancy. There’s nothing to worry about, especially if you work with an experienced OBGYN.

Somapika Dutta (B.Sc Physiology, Honours)
Somapikar holds Bachelors Degree in Physiology from University of Culcutta. She has 6+ years of experience writing in different niches, including health, tech and lifestyle. An animal enthusiast and a raging foodie, experiencing life - one day at a time.


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