How fast is your baby growing? See how fetal weight and height change by week during pregnancy

It's totally normal to wonder about your baby's size during pregnancy – and even to imagine how big your baby is using various fruits and vegetables. But don't expect your healthcare provider to give you official updates on the inches and ounces, because it's not something you need to follow closely.

"If all is going well, your baby will roughly match the numbers on the fetal growth chart, but it isn't something you'll routinely hear about during prenatal visits," says Layan Alrahmani, M.D., a board-certified ob-gyn and maternal-fetal medicine specialist and member of the BabyCenter Medical Advisory Board. "If you have questions about your baby's size and how they're growing, ask your provider for individual details."

Your doctor or midwife will keep track of your baby's growth and development using ultrasound and (later in pregnancy) your fundal height. If there are any concerns or you have a high-risk pregnancy, you'll have more ultrasounds to check on your baby's size. Using ultrasound, your provider can take various measurements (head circumference and diameter, abdomen circumference, and femur length) and use them to estimate your baby's size.

If all is going well, your baby will roughly match the numbers on the fetal growth chart, but it isn't something you'll routinely hear about during prenatal visits.

-  Layan Alrahmani, M.D, ob-gyn and maternal-fetal medicine specialist 

How do you determine fetus size by week?

There are different methods for estimating how big a fetus is, and honestly … it gets complicated.

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

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

Hadlock, the main source we use below, provides one of the most used – and to date most accurate – equations for estimating fetal height and weight. The American College of Obstetrics and Gynecology (ACOG) and the Society for Maternal and Fetal MedicineOpens a new window (SMFM) recommend using Hadlock's figures to diagnose and manage fetal growth conditions.

Note: 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 circumstances.

Using our fetal growth chart

The numbers in our chart below can give you a sense of your baby's size, but keep in mind that your pookie 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.

Boys' measurements are different from girls' 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 after that are from head to toe.

Fetal growth chart

Dr. Mark Curran, board-certified ob-gyn and maternal-fetal medicine specialist, helped create 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 growth will typically speed up until 35 weeks, then slow down.

Here are some highlights, based on estimations:

• Up until 16 weeks, your baby grows an average of about 19 grams per week, gradually increasing from 7 grams at 8 weeks to 15 grams at 12 weeks and 29 grams at 16 weeks.
• By 20 weeks, your baby is gaining about 59 grams per week (just over 2 ounces).
• By 30 weeks, your baby is gaining about 175 grams each week (more than 6 ounces).
• At 35 weeks, your baby 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, or about 6 ounces, 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) by week 40.

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 FGR 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.

Fetal height by week: How it changes

These numbers are tiny, but they add up quickly! Your baby will grow from about half an inch tall to a birth height averaging over 20 inches.

Here are some highlights, based on estimations:

• Your baby grows about 5 cm (almost 2 inches) from weeks 8 through 13.
• Your baby has a growth spurt between week 14 to about week 18, gaining almost 2 cm (.79 inches) each week.
• From weeks 19 through 23, your baby grows just over half an inch weekly.
• From week 24 through the end of pregnancy, your baby averages about 1 cm (.39 inches) in growth each week.

Your baby's size by week

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 12 ounces, and they're about the length of a banana.

At 32 weeks, your baby's lungs are developing fast, and your baby is storing minerals like iron for their first 6 months of life. Your baby's weight at 32 weeks is 4.3 pounds, about the size of a papaya, and their length is almost 17 inches.

At 33 weeks, your baby's skin is becoming less wrinkled as they fill in – they weigh almost 5 pounds. At just over 17 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 is about 6.7 pounds, and they're about the length of a bunch of Swiss chard, or 19 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% 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 about newborn weight gain and why it's important.

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