At 31 weeks of pregnancy, the average baby weighs about 3.3 pounds (1,502 grams). Your baby has finished most major development by this point and is now focused on gaining weight quickly over the remaining weeks.
Average Weight and Length at 31 Weeks
The 3.3-pound average is just that: an average. Healthy babies at 31 weeks can weigh noticeably more or less depending on genetics, placental function, and other factors. At this stage, babies are roughly 16 to 17 inches long from head to heel, though length measurements on ultrasound are less precise than weight estimates.
To put this in perspective, your baby will roughly double or even triple in weight between now and full term. The third trimester is the period of fastest weight gain, with babies typically adding about half a pound per week as they build up body fat. That fat layer serves two purposes: it helps regulate body temperature after birth and provides an energy reserve for the first days of life.
How Ultrasound Weight Estimates Work
If your provider gives you an estimated fetal weight at your 31-week appointment, it comes from ultrasound measurements of your baby’s head, abdomen, and thigh bone, which are plugged into a formula. These estimates are useful but not exact. A large study found that ultrasound weight estimates fell within 10% of the baby’s actual weight about 72% of the time. The average error was small (around 2%), but individual readings could be off by 10 to 20% in either direction. In roughly 3% of cases, the estimate was more than 20% off.
This means a baby estimated at 3.3 pounds could realistically weigh anywhere from about 2.9 to 3.6 pounds. The further a baby’s weight is from the middle of the range, the less accurate the estimate tends to be. If your provider flags a concern about your baby being too large or too small, they’ll typically repeat the measurement a few weeks later to look at the growth trend rather than relying on a single number.
What Affects Your Baby’s Weight
Several factors influence how much your baby weighs at any point in pregnancy:
- Genetics: Parental size is one of the strongest predictors. Taller or larger parents tend to have bigger babies, and this effect becomes more visible in the third trimester when growth rates diverge.
- Gestational diabetes: Higher blood sugar levels send extra glucose to the baby, which can accelerate weight gain. Babies of mothers with uncontrolled gestational diabetes often measure larger than average.
- Pre-pregnancy weight and weight gain: Your BMI before pregnancy and how much weight you’ve gained so far both play a role. The CDC recommends a total gain of 25 to 35 pounds for women who started at a normal weight, 15 to 25 pounds for those who were overweight, and 11 to 20 pounds for those with obesity.
- Parity: Second and subsequent babies tend to be slightly heavier than first babies at the same gestational age.
- Hypertensive disorders: Conditions like preeclampsia can reduce blood flow through the placenta, sometimes limiting how much nutrition reaches the baby and slowing growth.
None of these factors alone determines your baby’s weight. They interact with each other, which is why two women at the same gestational age can have babies that differ by a pound or more and both be perfectly healthy.
When Weight Falls Outside the Normal Range
Providers become concerned when a baby’s estimated weight drops below the 10th percentile for gestational age. This is sometimes called “small for gestational age.” A formal diagnosis of growth restriction isn’t based on a single measurement. It requires evidence that the baby’s growth rate has slowed over time, usually confirmed by comparing two ultrasounds taken weeks apart. If growth restriction is identified, your provider will monitor more frequently to check blood flow through the umbilical cord and assess the baby’s well-being.
On the other end, babies measuring above the 90th percentile are considered large for gestational age. This is more common with gestational diabetes but can also happen in pregnancies with no identified risk factors. A large estimate at 31 weeks doesn’t necessarily predict a large baby at birth, partly because of the margin of error in ultrasound measurements and partly because growth rates can change in the final weeks.
What Else Is Happening at 31 Weeks
Weight is just one piece of the picture at this stage. Your baby’s lungs are maturing rapidly but aren’t fully ready for breathing air on their own. The brain is developing quickly too, forming billions of connections that will continue building well after birth. Your baby can now process light and sound, and you may notice responses to loud noises or bright lights.
Most babies haven’t settled into their final birth position yet. About 1 in 5 babies are still in a breech position (feet or bottom down) at 30 weeks. This is normal. The majority will turn head-down on their own before 36 to 37 weeks, so a breech position at your 31-week check is not a cause for concern.
Your baby’s movements should feel strong and regular by now. Kicks, rolls, and stretches are a good sign that things are progressing well, regardless of where your baby falls on the weight chart.