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Fetal Biometry: Measurements, Normal Values and Gender Prediction

Updated on: September 9, 2024 | PocketHealth
A pregnant patient looking at an ultrasound image on the phone

Throughout pregnancy, you’ll encounter terms like fetal biometry, BPD, OFD and others. But what do they indicate about your baby’s health? Here’s what you need to know.

Pregnancy ultrasounds provide a wealth of information about your baby’s growth. Fetal biometry measurements are essential for monitoring this development, offering a window into your baby’s health and progress. In this comprehensive guide, we will explore the various measurements that define fetal biometry, what they indicate about your baby’s health, and how they can be used to predict gestational age and track growth patterns. Whether you’re navigating your first pregnancy or seeking to deepen your understanding, this article will equip you with knowledge to confidently interpret these measurements.

 

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What is fetal biometry?

Fetal biometry is a technique that uses ultrasound to measure the size and growth of a fetus during pregnancy. It involves several measurements that assess growth progress, compare proportions to standardized baseline markers and calculate size percentiles.  These measurements provide insights into the fetus’s development and help healthcare providers monitor gestational age and detect any potential growth abnormalities. Throughout your pregnancy, you will likely have multiple ultrasound scans to track these metrics.

Because fetal biometry can be quite technical, using PocketHealth Report Reader can help clarify the medical terminology in your ultrasound report. This tool allows you to better understand and track the progression of your scans.

 

Why is fetal biometry important?

Fetal biometric parameters are important because they compare the baby’s size to their gestational age, helping to estimate how many weeks along in the pregnancy they are. If there are fetal growth abnormalities, such as a statistically significant size difference compared to the typical range, further testing may be required.

Some possible reasons for these size abnormalities can include placental issues, congenital fetal anomalies, or other health complications for the fetus or the expectant parent. Nevertheless, there are several common reasons for a fetus to measure smaller or larger, and it is not necessarily a cause for concern. Humans come in all sizes, even in the womb. Your doctor will determine any additional steps or tests to take, if needed.

Multiple standard measurements are used to assess fetal growth, providing a baseline for common parameters such as BPD, HC, FL, and others as necessary. Additional measurements from the BPD ultrasound can clarify anomalies or fetal growth disorders, enabling your doctor to closely monitor specific areas and track progression. You can access your reports through PocketHealth, which allows you to view these metrics and provides valuable data for discussion with your healthcare provider.

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Ultrasound Results

 

Standard measurements: What do BPD, HC, FL and AC mean on an ultrasound?

Fetal ultrasound showing BPD, FC, HC and other biometric measurements

Fetal ultrasound showing biometric measurements (BPD, HC, FL and others)

Usual fetal biometry measurements are taken during standard ultrasounds, though some metrics may not be accurate until later in the pregnancy. Typically, this data is collected around 13 weeks gestation or later, save for an exception or two. BPD, HC, FL and AC are considered standard fetal biometric measurements. 

What is BPD on an ultrasound?

BPD or Biparietal Diameter, is a key measurement that is taken during routine pregnancy ultrasounds. It is a measurement of the distance between the two sides of the baby’s head—specifically, the space between the left and right parietal bones of the skull. This measurement is crucial for several reasons: 

  • Estimating gestational age and fetal weight: BPD is commonly used by doctors to estimate the gestational age of the fetus. Since the fetal head grows at a predictable rate during pregnancy, the BPD provides a reliable metric to determine how far along the pregnancy is. Along with other measurements, such as femur length and abdominal circumference, BPD also helps estimate the weight of the fetus.
  • Assessing fetal development: Monitoring BPD over time allows doctors to assess the progress of fetal development. A steady increase in BPD is a positive sign, indicating normal growth patterns. On the other hand, deviations from the expected range may raise concerns. For instance, a BPD that is significantly smaller or larger than expected could indicate potential growth issues or conditions such as microcephaly or macrocephaly, respectively.

Accurate BPD measurements are essential for effective prenatal care. They guide medical decisions, from determining the due date to identifying potential complications early on. In cases where abnormalities are detected, further testing and monitoring may be necessary to ensure the health and well-being of both the mother and the baby.

Ultrasound showing BPD and HC

Ultrasound image showing BPD and HC

What are HC, FL and AC on an ultrasound?

In addition to the Biparietal Diameter (BPD), several other measurements are taken during a standard pregnancy ultrasound to assess fetal growth and development. These include Head Circumference (HC), Femur Length (FL) and Abdominal Circumference (AC). Each of these measurements provides valuable insights into the health and development of the fetus.

  • HC (Head circumference) : HC measures the circumference of the baby’s head, providing an overall assessment of head size. This measurement is crucial for monitoring brain development and identifying potential abnormalities in head growth. Along with BPD, HC helps in accurately estimating gestational age. Consistent measurements throughout the pregnancy ensure that the baby’s head is growing at a normal rate, which is essential for overall fetal health.
  • FL (Femur length): FL refers to the length of the baby’s femur, the longest bone in the body. Measuring the femur’s length gives healthcare providers a clear picture of the fetal skeletal development and overall growth. FL is particularly important in estimating the fetal length and weight, which are key indicators of fetal health. Abnormal FL measurements may signal growth restrictions or other developmental issues, necessitating further examination.
  • AC (Abdominal circumference): AC is the measurement of the fetal abdomen’s circumference. This measurement is a critical indicator of fetal nutrition and growth, as the size of the abdomen reflects the development of vital organs such as the liver, stomach, and intestines. AC is often used in combination with HC and FL to estimate fetal weight and to identify any potential growth abnormalities. A disproportionate AC, whether too large or too small, can indicate issues such as gestational diabetes or intrauterine growth restriction (IUGR).

HC, FL and AC play a crucial role in evaluating fetal well-being. They are key to accurately determining the due date, assessing fetal health, and guiding decisions throughout the pregnancy.

Additional measurements: CRL, OFD, HL and others

Fetal ultrasound showing the crown rump length (CRL) measurement

Fetal ultrasound showing the crown rump length (CRL) measurement

Additional biometry measurements are used to determine accurate gestational age, check for physical abnormalities or growth disorders and estimate fetal weight, which is useful for tracking development.

  • CRL (Crown-rump-length): Measurement of the torso length from the top of the head to the baby’s bottom. This is commonly most accurate before 12 weeks of gestational age and is one of the earliest metrics taken.
  • HL (Humeral length): Measurement of the humerus bone length, typically measured during the second trimester. Short lengths can sometimes be an indicator for Down syndrome and other conditions.
  • TCD (Trans-cerebellar diameter): Measurement of the diameter between the cerebellar hemispheres, a reliable way to confirm gestational age.
  • FP (Fetal pyelectasis): Measurement of the diameter of the renal pelvis to check for enlargement and the health of the kidneys, ureter, etc. Enlargement is usually found in the second trimester, but the issues frequently resolve themselves.
  • IOD (Interocular distance): Measurement of the area between the two medial canthi of both eyes. Abnormal values could indicate congenital syndromes.
  • OFD (Occipital-frontal diameter): Measurement of the area between the fetus’s forehead and the back of the head. This is usually assessed in the second and third trimesters and helps predict head shape.
  • BOD (Binocular distance): Measurement of the area between the two outer canthi of the eyes.
  • Ratios: Various ratios are used to ascertain normal fetal growth patterns. These include the thoracic to abdominal circumference ratio, the thoracic to head and the cardio-thoracic circumference ratios.

Ultrasound scan to measure BPD, HC and other parameters

The ultrasound scan provides fetal biometric measurements, including important data on BPD, HC, and other growth standards. This information can aid in prenatal diagnosis of intrauterine growth restriction or other abnormal growth conditions, enabling healthcare providers to intervene early.

How is the test performed?

Fetal biometry measurements are taken during standard prenatal ultrasound screenings. Ultrasounds utilize high-frequency sound waves that bounce off internal structures and organs, creating images from these echoes. Technicians use these images to check anatomy, perform an amniotic fluid volume assessment and record fetal biometry dimensions.

Most of your pregnancy scans will be abdominal ultrasounds. During the procedure, the technician applies clear gel to your abdomen and moves a small transducer over the area to capture images. Early pregnancy scans or those requiring specific views may be performed internally by inserting an ultrasound wand 2-3 inches into your vagina. While this might feel slightly uncomfortable, it should not be painful. To improve imaging quality, your healthcare provider may ask you to have a full bladder for both types of scans. Be sure to request any preparatory instructions ahead of time.

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When is the test performed?

Fetal biometry data will be collected during all of your standard ultrasounds. Most pregnancies require only two or three ultrasounds: one at 6-8 weeks for gestational dating, another during the second trimester for anatomy scans and sometimes a third near the end of pregnancy to check the baby’s position. High-risk patients or those carrying multiples may have additional scans.

How accurate are the measurements?

During the first and second trimesters, fetal biometry is an accurate and dependable way to evaluate fetal growth. Accuracy declines somewhat in the third trimester, as fetal size begins to vary and growth becomes more individualized. As with any type of exam, errors from the machine or technician can occur, but overall, these scans are considered very reliable, especially when combined with other forms of testing.

 

Normal fetal biometry values: BPD, HC, AC, FL week-by-week chart

The chart below shows the average values of BPD, FL, HC, and AC for specific gestational ages. However, it is common to see variation in fetal biometric measurements due to factors such as parental sizes, medical conditions, or genetic factors.

Gestational Age (weeks)

BPD (mm)

FL (mm)

HC (mm)

AC (mm)

11

15

08

48

49

14

24

12

80

74

16

32

14

104

120

18

38

23

132

125

20

49

34

170

152

22

57

42

208

196

23

53

48

230

206

24

66

49

240

220

26

71

54

258

250

28

79

59

283

260

30

80

63

298

284

32

89

70

320

299

However, it’s important to recognize that these values can vary significantly from one pregnancy to another. Factors such as parental size, maternal health conditions, and genetic influences can all contribute to differences in fetal biometric measurements. For instance, larger parents may naturally have fetuses with slightly higher measurements, while certain medical conditions could result in smaller or larger-than-average values.

While these variations are often within the range of normal development, consistent monitoring is essential to ensure the fetus is growing as expected. Any significant deviations from these averages might prompt further investigation to rule out growth disorders or other potential concerns.

 

When will I get my ultrasound report?

A pregnant person looking at their pregnancy ultrasound report on PocketHealth

A pregnant person looking at their pregnancy ultrasound report on PocketHealth

Typically, most patients receive their ultrasound reports at their next prenatal doctor’s visit. For quicker access, you can use PocketHealth to easily and securely access your images and report as soon as it is interpreted and officially released. This allows you to review your results before seeing your doctor and to be better prepared.

If you need assistance with questions to ask, MyCare Navigator provides personalized suggestions to help maximize your time with your provider. Here are some examples of questions you may want to ask:

  • How often will I get ultrasound scans?
  • How long will it be to get my results?
  • When will I know my baby’s gender?
  • What preparations should I make before my ultrasound?

Understanding your results

Your referring physician will discuss your ultrasound results with you at your next appointment. In the meantime, PocketHealth can assist you in understanding your report while you wait to see your healthcare provider. PocketHealth Report Reader provides definitions for medical terminology found in your report, enhancing your understanding of your results.

While PocketHealth does not replace professional medical advice, it serves as a valuable tool to empower patients in accessing and understanding their health. It can also help prepare you for a more productive appointment with your doctor.

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Ultrasound Results

 

Frequently asked questions

Here are some specifics on fetal biometry data and questions you may have.

Can fetal biometry predict gender?

Gender-detecting ultrasounds commonly occur between gestational weeks 18-20, when fetal anatomy has developed enough for accurate visibility. However, fetal ultrasound biometry

can predict gender earlier. Male and female fetuses develop different growth patterns in measurements like CRL and HC. The accuracy of these predictions increases with gestational age. Gender can usually be determined when the crown-rump-length is at or over 60 mm; if it is below this measurement, then it is too early for an accurate prediction.

What percentile fetal size is a healthy baby?

Typically, medical professionals are not concerned about fetal size unless it falls below the 10th percentile or above the 90th percentile of average measurements, which may indicate growth-restricted fetuses or abnormal growth beyond expected norms. However, there are numerous exceptions to this rule, as fetal weight and size can vary widely. Your doctor will make determinations based on your individual health and medical history.

What percentile in fetal size is concerning?

If the fetus is below the 10th percentile for their gestational age, it can warrant further evaluation. This lower size is referred to as fetal growth restriction (FGR) and may necessitate additional fetal monitoring. Remember, there are many reasons why a fetus may measure small, so try not to panic. Discuss any necessary next steps  with your doctor.

What does an abnormal result mean?

Atypical ultrasound results will likely require further monitoring. Tracking the fetus’s development helps identify trends so your provider can make appropriate recommendations. Fetal growth restriction can result from placental or maternal factors, among other possibilities. Large size may indicate temporary gestational diabetes in the pregnant person, which is common and usually manageable. There are numerous potential reasons for abnormal sizing, so try not to be alarmed. Your doctor will help identify the cause and recommend any necessary actions.

Will an early ultrasound show fetal biometry?

Typically, your first ultrasound examination is done between 10-14 weeks to assess fetal size, ensure there are no pregnancy complications, and examine the baby’s anatomy. However, early ultrasound screenings are also quite common, often occurring at 6-7 weeks into the pregnancy. These early scans may not provide complete measurements of HC, AC and FL, but they do measure CRL. They are also important for gestational age estimation and confirming conception dating. Additional fetal biometry data becomes visible as the fetus matures.

Stay on Top of Your Pregnancy Journey with PocketHealth

PocketHealth allows you to access your imaging records and reports in one place, making it convenient to share critical information with your medical team. Access to your pregnancy ultrasounds is particularly valuable, as you may need to show these records to specialists, primary care providers, or other healthcare professionals. Having these images for personal use is also beneficial, as family and friends may want to follow the progress of your pregnancy.

 

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