Fetus size by week: Your baby's weight throughout pregnancy (2024)

How do you determine fetus size by week?

There are different methods for estimating how big a fetus is, which is why you'll find different numbers depending on the source.

Experts have formulas they use to come up with the estimated fetal weight (EFW) and height of a fetus, and the formulas aren't always the same. The measurements that are used in equations to estimate weight usually include biparietal (head) diameter (BPD), head circumference (HC), abdominal circumference (AC) and femur (thigh bone) length (FL).

Height is a straightforward measurement, but the method of measuring it changes after the first trimester. For the first 13 weeks, the height measurement is taken from the top of the head to the baby's bottom. After the first 13 weeks, the measurement is taken from the top of the head to the baby's heel – explaining why, on the chart below, your baby appears to grow 3 inches from week 13 to week 14!

Hadlock, the main source we use in our fetal growth chart, provides one of the most commonly used – and most accurate – equations for estimating fetal height and weight. The American College of Obstetrics and Gynecology (ACOG) and the Society for Maternal and Fetal Medicine (SMFM) use Hadlock's figures to diagnose and manage fetal growth conditions, such as intrauterine growth restriction (IUGR).

The numbers on our chart should coincide with the numbers your healthcare provider will be checking against when they measure your baby using ultrasound. (Providers don't measure height after 13 weeks, however, so don't expect to get those numbers at your ultrasound appointments.)

Note that the data used by Hadlock was gathered from middle-class Caucasian women with no history of maternal diseases known to affect fetal growth and no evidence of congenital anomalies. Your provider may make adjustments based on your individual circ*mstances.

Fetal growth chart

Wondering how big your baby is during each week of pregnancy? The numbers in our chart below can give you a sense of your baby's size. Keep in mind that your baby may be much smaller or larger than these averages. That's okay – after all, healthy babies can weigh less than 5 pounds or more than 9 pounds at birth.

Boy's measurements are different than girl's measurements, even this early. For the numbers on our chart, we've taken an average of boys and girls. And remember, the height measurements up to 13 weeks are head-to-bottom estimates, while the height measurements starting at week 14 are head-to-toe estimates.

Thanks to Dr. Mark Curran, maternal-fetal medicine specialist, for his help preparing this chart.

Fetal weight by week: How it changes

Your baby steadily gains weight over the course of your pregnancy, but it's not always at the same rate. If you're having one baby (not twins or multiples), your baby's rate of growth accelerates until 35 weeks, then decelerates.

Here are some highlights, based on estimations:

• Up until 16 weeks, a fetus grows an average of about 19 grams per week, gradually increasing from 7 grams per week at 8 weeks to 15 grams per week at 12 weeks and 29 grams per week at 16 weeks.
• By 20 weeks, a fetus is gaining about 59 grams per week (just over 2 ounces).
• By 30 weeks, a fetus is gaining about 175 grams each week (more than 6 ounces).
• At 35 weeks, a fetus is gaining about 215 grams each week, or about 7.5 ounces. At this point their growth rate peaks.
• After 35 weeks, growth slows to about 188 grams per week, or 6.6 ounces. (Twins slow earlier, at around 28 weeks, and then average about 170 grams each week.)
• In the last few weeks of pregnancy, the growth rate continues to gradually slow to about 168 grams (a little less than 6 ounces) per week by week 40.

Using a tape measure stretched over your belly, your provider will use a fundal height measurement to check your baby's size at your prenatal visits. Beginning at about 24 weeks, the measurement in centimeters should roughly match the gestational age of your baby. If you're 26 weeks pregnant, for example, your fundal height should be about 26 cm, give or take a centimeter in each direction.

If your provider is concerned that your baby is too small, they'll monitor your baby's size with ultrasound, which is more accurate. Using ultrasound, your practitioner can take various measurements (head circumference and diameter, abdomen circumference, femur length) and use them to estimate your baby's size. They may also use a Doppler ultrasound to look at the blood flow to your placenta.

If your baby's estimated weight is less than the 10th percentile for their gestational age, they may be diagnosed with intrauterine growth restriction (IUGR), also called fetal growth restriction (FGR). IUGR can happen at any time during pregnancy. Some babies with IUGR just turn out to be small for their age, but sometimes there's a problem that's preventing the baby from growing properly.

At birth, a baby with IUGR is called "small for gestational age." While most SGA babies who are otherwise healthy grow just fine, some (especially those born prematurely) are at higher risk of problems such as c-section, jaundice, low blood sugar, and even long-term developmental and health problems.

Your baby's size by week

Here are some highlights of your baby's growth during pregnancy:

At 20 weeks, about the midpoint in your pregnancy, your baby is transmitting taste signals to their brain. And you may feel them hiccupping. Your baby's weight at 20 weeks is about 11.68 ounces, and they're about the length of a (10.12-inch) banana.

At 32 weeks, your baby's lungs are developing fast, and your baby's storing minerals like iron for their first 6 months of life. Your baby's weight at 32 weeks is 4.30 pounds, and their length is 16.93 inches, about the size of a jicama.

At 33 weeks, things are getting snug in there! Your baby's skin is becoming less wrinkled as they fill in – your baby's weight at 33 weeks is about 4.77 pounds. At 17.36 inches, your baby is now about the size of a pineapple.

At 37 weeks, your baby's brain and lungs are still maturing, and they're still moving a lot, despite the close quarters. Your baby's weight at 37 weeks is about 6.68 pounds, and they're about the length of a bunch of Swiss chard, 19.02 inches.

Once your baby is born, they'll be weighed and measured, and your provider will continue to monitor their growth. While the average newborn weight is a little over 7 pounds, most newborns lose about 5 to 10 percent of their weight in the first days. No worries – they gain it back by the time they're about 2 weeks old, and by 4 months they usually double their birth weight.

Learn more:

• To-do lists for the first, second, and third trimesters
• Pregnancy Due Date Calculator
• Pregnancy Weight Gain Calculator
• How to understand pregnancy weeks, months, and trimesters