Fetal Femur Length by Week: What Moms-to-Be Should Know

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Fetal femur length indicates fetal development. However, many mothers often overlook this standard during prenatal care. Let’s explore this issue with AVAKids in the article below!

Important ultrasound measurements

Each period marks a transition for both mother and baby. Fetal biometrics will show the baby’s condition at each stage. In the ultrasound image, you will see some abbreviations. AVAKids will explain these abbreviations:

  • GA (Gestational age): Calculated from the first day of the last menstrual period before 12 weeks. After this point, when the estimated due date (EDD) is determined by the first-trimester ultrasound, gestational age is calculated based on the EDD.
  • GSD (Gestational Sac Diameter): The diameter of the gestational sac. At this stage, the fetus has not yet formed organs but is developing within the gestational sac.
  • BPD (Biparietal diameter): The diameter of the fetal head at its widest point.
  • FL (Femur length): The length of the fetal femur (thigh bone).
  • EFW (Estimated fetal weight): The estimated weight of the fetus based on BPD, FL, AC (abdominal circumference), and HC (head circumference).
  • CRL (Crown rump length): The length from the top of the baby’s head to its bottom. During the first half of pregnancy, the fetus is often curled up, making it difficult to accurately measure the full length. Therefore, in the later weeks, CRL is replaced by the full fetal length measurement. When the fetal measurements reach a certain limit, the mother should observe and recognize the signs of impending labor and wait for the baby’s arrival.

What does fetal femur length mean?

The significance of measuring fetal femur length is similar to other measurements. Femur length by gestational week reflects the baby’s development. If the femur length corresponds to the normal range in the standard chart, the mother can rest assured and await the baby’s arrival.

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Standard fetal femur length chart

From the 14th week of pregnancy, doctors begin to measure the fetal femur length during ultrasounds. Below is a standard femur length chart that you can refer to, based on the International Fetal Growth Standards from the University of Oxford.

Gestational Age (Weeks) Average Value (mm)
Limit Range (mm)
Week 14 14 13 – 15
Week 15 17 16 – 19
Week 16 20 18 – 22
Week 17 23 22 – 26
Week 18 25 25 – 29
Week 19 28 27 – 33
Week 20 31 30 – 36
Week 21 34 32 – 38
Week 22 36 35 – 41
Week 23 39 37 – 45
Week 24 42 40 – 48
Week 25 44 42 – 50
Week 26 47 45 – 53
Week 27 49 46 – 56
Week 28 52 49 – 59
Week 29 54 51 – 61
Week 30 56 53 – 63
Week 31 59 55 – 65
Week 32 61 56 – 68
Week 33 63 58 – 70
Week 34 65 60 – 72
Week 35 67 62 – 74
Week 36 68 64 – 76
Week 37 70 66 – 79
Week 38 71 67 – 81
Week 39 73 68 – 72
Week 40 74 70 – 84

Fetal femur length chart

Monitoring fetal measurements weekly is recommended to equip mothers with basic knowledge about these indicators, helping them understand their baby’s development.

Is a shorter-than-standard fetal femur length a concern?

In some cases, an ultrasound may show a short femur length, which is considered a marker for Down syndrome. However, this is just a marker, not a definitive diagnosis. The best way to assess whether a fetus has Down syndrome is by measuring nuchal translucency or based on the mother’s medical history or family genetic factors. Furthermore, determining whether the fetal femur length meets the standard also depends on bone mineral density and other parameters. Further testing may be necessary for a more accurate diagnosis.

Factors affecting fetal femur length

There are four main factors affecting femur length:

Genetics

Genetics is one of the factors affecting fetal femur length, accounting for up to 23%. If the fetus’s femur length is influenced by genetics, it is difficult to change.

Genetic factors affecting fetal femur length

Genetic factors affecting fetal femur length

Cerebellar hypoplasia

Cerebellar hypoplasia in the fetus can affect femur length. In this case, if the pregnant woman experiences any unusual abdominal pain, she should immediately seek examination at a reputable hospital or clinic.

Maternal nutrition

During pregnancy, a mother’s diet needs to be more strictly adhered to than usual. Following outdated nutritional beliefs can significantly impact the fetus’s femur length. For example, if a mother focuses too much on protein intake while neglecting milk and other calcium-rich foods, it can negatively affect bone development. In addition, pregnant women also need to supplement other essential nutrients such as vitamin D, calcium, protein, iodine, iron, folic acid, and unsaturated fatty acids (DHA, ARA) for better fetal femur development.

Lifestyle habits

A pregnant woman’s use of stimulants or unhealthy habits like staying up late, drinking a lot of soft drinks, coffee, alcohol, or smoking can also indirectly affect the baby’s healthy development. Even secondhand smoke can have a significant impact on the fetus.

How to help the fetus develop healthily

For normal fetal development, mothers should pay attention to their nutrition and maintain a healthy lifestyle by:

  • Getting enough sleep, exercising regularly with moderate activity, and eating a healthy and balanced diet.
  • Consuming kale, spinach, broccoli, and bok choy. These dark green vegetables provide fiber, calcium, vitamin A, and potassium.
  • Increasing intake of almonds, walnuts, sesame seeds, and beans for calcium.
  • Regularly eating calcium-rich seafood like loach, sardines, shellfish, oysters, shrimp, prawns, crabs, and small fish, including the bones (cooked thoroughly) for extra calcium.
  • Consuming protein-rich foods like milk, salmon, mushrooms for vitamin D, meat, fish, and eggs for protein.
  • Adding folic acid from broccoli, beans, and parsley.
  • Drinking one to two glasses of maternity milk, yogurt, or eating cheese daily.
  • Limiting processed foods with preservatives and fried foods.
  • Completely avoiding tobacco, alcohol, and coffee.
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