Estimation of Fetal Weight
From early in pregnancy, babies grow at different rates, so these numbers are merely averages. Your baby’s actual length and weight may vary substantially.
Don’t worry too much if an ultrasound indicates that your baby is much smaller or larger. (Your practitioner will let you know if it’s time to worry about how big your baby is.) By full-term, your baby may end up weighing less than 5 pounds or more than 9.
Until about 20 weeks, babies are measured from the crown (or top) of the head to the rump (or bottom). This is because a baby’s legs are curled up against his torso during the first half of pregnancy and very hard to measure.
After that, babies are measured from head to toe.
See our pregnancy timing article to learn how the weeks of pregnancy are counted and your baby’s gestational age is determined. If you don’t know your due date, use our due date calculator to find out.
|Gestational age||Length (US)||Weight (US)||Length (cm)||Mass (g)|
|(crown to rump)||(crown to rump)|
|8 weeks||0.63 inch||0.04 ounce||1.6 cm||1 gram|
|9 weeks||0.90 inch||0.07 ounce||2.3 cm||2 grams|
|10 weeks||1.22 inch||0.14 ounce||3.1 cm||4 grams|
|11 weeks||1.61 inch||0.25 ounce||4.1 cm||7 grams|
|12 weeks||2.13 inches||0.49 ounce||5.4 cm||14 grams|
|13 weeks||2.91 inches||0.81 ounce||7.4 cm||23 grams|
|14 weeks||3.42 inches||1.52 ounce||8.7 cm||43 grams|
|15 weeks||3.98 inches||2.47 ounces||10.1 cm||70 grams|
|16 weeks||4.57 inches||3.53 ounces||11.6 cm||100 grams|
|17 weeks||5.12 inches||4.94 ounces||13 cm||140 grams|
|18 weeks||5.59 inches||6.70 ounces||14.2 cm||190 grams|
|19 weeks||6.02 inches||8.47 ounces||15.3 cm||240 grams|
|20 weeks||6.46 inches||10.58 ounces||16.4 cm||300 grams|
|(crown to heel)||(crown to heel)|
|20 weeks||10.08 inches||10.58 ounces||25.6 cm||300 grams|
|21 weeks||10.51 inches||12.70 ounces||26.7 cm||360 grams|
|22 weeks||10.94 inches||15.17 ounces||27.8 cm||430 grams|
|23 weeks||11.38 inches||1.10 pound||28.9 cm||501 grams|
|24 weeks||11.81 inches||1.32 pound||30 cm||600 grams|
|25 weeks||13.62 inches||1.46 pound||34.6 cm||660 grams|
|26 weeks||14.02 inches||1.68 pound||35.6 cm||760 grams|
|27 weeks||14.41 inches||1.93 pound||36.6 cm||875 grams|
|28 weeks||14.80 inches||2.22 pounds||37.6 cm||1005 grams|
|29 weeks||15.2 inches||2.54 pounds||38.6 cm||1153 grams|
|30 weeks||15.71 inches||2.91 pounds||39.9 cm||1319 grams|
|31 weeks||16.18 inches||3.31 pounds||41.1 cm||1502 grams|
|32 weeks||16.69 inches||3.75 pounds||42.4 cm||1702 grams|
|33 weeks||17.20 inches||4.23 pounds||43.7 cm||1918 grams|
|34 weeks||17.72 inches||4.73 pounds||45 cm||2146 grams|
|35 weeks||18.19 inches||5.25 pounds||46.2 cm||2383 grams|
|36 weeks||18.66 inches||5.78 pounds||47.4 cm||2622 grams|
|37 weeks||19.13 inches||6.30 pounds||48.6 cm||2859 grams|
|38 weeks||19.61 inches||6.80 pounds||49.8 cm||3083 grams|
|39 weeks||19.96 inches||7.25 pounds||50.7 cm||3288 grams|
|40 weeks||20.16 inches||7.63 pounds||51.2 cm||3462 grams|
|41 weeks||20.35 inches||7.93 pounds||51.7 cm||3597 grams|
|42 weeks||20.28 inches||8.12 pounds||51.5 cm||3685 grams|
Where to go next:
Fetal ultrasound measurements show how the baby is growing and also help detect abnormalities. The estimation of fetal weight during pregnancy is among the most important examinations done. These measurements may help your doctor determine whether the baby is too small (intrauterine growth restriction: IUGR) or too big (large for gestational age: LGA).
Babies that are too small or too large have higher risks of complications. Because the early detection of growth abnormalities may help to manage complications more appropriately even before the baby is born, monitoring fetal growth is an important part of antepartum care.
Monitoring can be done through several steps including palpating the uterus and the fetus, measuring the size of the uterus, and performing a sonogram. A sonogram will measure various parts of the fetus, including the head, abdomen, and upper thighbone.