Your baby’s size changes dramatically from week to week during pregnancy, starting smaller than a grain of rice and reaching an average of about 7.5 to 8 pounds by the time you deliver. At 12 weeks, the baby measures roughly 2 inches long and weighs half an ounce. By 20 weeks, that jumps to about 6.5 inches and 10.5 ounces. Here’s what to expect at every stage.
First Trimester: Weeks 9 Through 12
During the first trimester, your baby is measured from the top of the head to the bottom of the torso (called crown-to-rump length) because the legs are too small and curled to measure reliably. Growth is rapid. At 9 weeks, the average length is about 0.6 inches and the baby weighs roughly 2 grams, less than a paperclip. By 10 weeks, the baby has nearly doubled to 1.2 inches and 4 grams.
At 11 weeks, your baby reaches about 1.6 inches and 7 grams. By week 12, they measure around 2.1 inches and 14 grams, roughly half an ounce. An international study of over 4,000 pregnancies found the average crown-to-rump length at 12 weeks was about 6 centimeters (2.4 inches), with normal variation of plus or minus 0.7 centimeters. That range matters: two healthy 12-week babies can differ in length by more than half an inch and both be perfectly on track.
Second Trimester: Weeks 13 Through 27
The second trimester is when your baby transitions from measuring in grams to measuring in pounds, and the growth is striking week by week.
At 13 weeks, the baby is about 2.2 inches and 23 grams (just under an ounce). By 16 weeks, they’ve roughly doubled in length to 4.6 inches and quadrupled in weight to about 100 grams, or 3.5 ounces. At 20 weeks, the midpoint of pregnancy, your baby measures around 6.5 inches from crown to rump and weighs about 300 grams (10.5 ounces).
Starting around week 21, measurements shift from crown-to-rump to head-to-heel, which is why you’ll see a big jump in the numbers. At 21 weeks, the head-to-heel length is about 10.5 inches and the baby weighs roughly 360 grams (12.7 ounces). By 24 weeks, the end of the second trimester, your baby stretches to nearly 12 inches and weighs about 600 grams, which is 1.3 pounds.
Third Trimester: Weeks 28 Through 40
The third trimester is all about weight gain. Your baby’s length increases steadily, but the real action is in how quickly they fill out.
At 28 weeks, the baby is about 14.8 inches long and weighs roughly 1,005 grams (2.2 pounds). By 32 weeks, they’ve grown to about 16.2 inches and 1,702 grams (3.75 pounds). At 36 weeks, expect roughly 18.7 inches and 2,622 grams (5.8 pounds).
In the final weeks, your baby puts on about half a pound per week. At 40 weeks, the average estimated weight is 3,617 grams, which works out to just under 8 pounds. The normal range is wide: a baby at the 10th percentile weighs about 6.8 pounds at 40 weeks, while a baby at the 90th percentile weighs around 9.1 pounds. Both are considered healthy, appropriate sizes.
What Your Provider Measures at Appointments
After about 20 weeks, your provider will likely start measuring your fundal height at each visit. This is the distance from your pubic bone to the top of your uterus, measured with a simple tape measure. The rule of thumb: the measurement in centimeters should roughly match how many weeks pregnant you are, plus or minus 2 centimeters. So at 32 weeks, anything from 30 to 34 centimeters is considered normal.
On ultrasound, your provider looks at four key measurements to estimate the baby’s size: the diameter across the head, the circumference around the head, the circumference around the belly, and the length of the thigh bone. At 20 weeks, for example, the average head circumference is about 17.5 centimeters. These measurements get plugged into a formula that generates an estimated weight. Keep in mind that ultrasound weight estimates can be off by 10 to 15 percent in either direction, especially later in pregnancy.
What Affects Your Baby’s Size
Genetics, sex, and your body type together account for roughly 40% of the variation in birth weight. That’s a huge proportion, which means two healthy pregnancies with perfect nutrition and prenatal care can still produce babies that differ by two or more pounds at birth. If you and your partner were both large babies, yours is more likely to be on the bigger side too.
Your pre-pregnancy weight plays a measurable role. People who start pregnancy with a BMI under 20 have a significantly higher chance of having a smaller baby compared to those with a BMI above 25. Blood pressure complications like preeclampsia also have a strong association with lower birth weight, as do certain pregnancy complications.
Consistent prenatal care makes a real difference. One study found that people who didn’t receive regular prenatal visits were seven times more likely to have a low-birth-weight baby compared to those who did. This likely reflects a combination of factors, including nutrition monitoring, early detection of complications, and general health support throughout pregnancy.
How Twin Growth Differs
If you’re carrying twins, their growth tracks closely with singleton babies through the first two trimesters. The divergence starts around 26 to 28 weeks, when twins typically begin growing more slowly than a single baby would. By the late third trimester, twins are often noticeably smaller than singleton averages at the same gestational age. This is a normal pattern, not a sign of a problem, and providers use twin-specific growth charts to assess whether each baby is on track.
Quick Reference: Size by Month
- Month 3 (weeks 9 to 12): 0.6 to 2.1 inches, 2 to 14 grams
- Month 4 (weeks 13 to 16): 2.2 to 4.6 inches, 23 to 100 grams
- Month 5 (weeks 17 to 20): 5.1 to 6.5 inches, 140 to 300 grams
- Month 6 (weeks 21 to 24): 10.5 to 11.8 inches, 360 to 600 grams
- Month 7 (weeks 25 to 28): 13.6 to 14.8 inches, 1.5 to 2.2 pounds
- Month 8 (weeks 29 to 32): 15.2 to 16.2 inches, 2.5 to 3.75 pounds
- Month 9 (weeks 33 to 36): 17.2 to 18.7 inches, 4.2 to 5.8 pounds
- Weeks 37 to 40: 19 to 20+ inches, 6.5 to 8+ pounds