From early on 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. Note that babies are measured from the crown (or top) of the head to the rump (or bottom) until about 20 weeks.

After that, they're measured from head to toe. (This is because a baby's legs are curled up against his torso during the first half of pregnancy and very hard to measure.)

See our help section if you have questions about how we count the weeks of pregnancy and gestational age.

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
43 weeks 20.20 inches 8.19 pounds 51.3 cm 3717 grams



Sources:

Doublet PM, Benson CB, Nadel AS, et al: "Improved birth weight table for neonates developed from gestations dated by early ultrasonography." Journal of Ultrasound Medicine. 16:241, 1997.

Hadlock FP, Shah YP, Kanon DJ, et al. "Fetal crown rump length: Reevaluation of relation to menstrual age with high resolution real-time US Radiology." 182:501, 1992.

Usher R, McLean F. "Intrauterine growth of live-born Caucasian infants at sea level: Standards obtained from measurements in 7 dimensions of infants born between 25 and 44 weeks of gestation." Pediatrics. v.74, 1969.

Wigglesworth JS. Perinatal Pathology, Second Edition. W.B. Saunders Company. 1996. page 24.