At 31 weeks of pregnancy, a baby typically weighs about 1,500 grams, or roughly 3.3 pounds. That’s about the size of a coconut, measuring around 16 inches from head to toe. This is a ballpark figure, though, and healthy babies at this stage can weigh somewhat more or less.
What 3.3 Pounds Looks Like
At 31 weeks, your baby has entered one of the most rapid growth phases of pregnancy. The third trimester is when babies pack on fat beneath their skin, filling out from a lean, wrinkled appearance into the rounder shape you’ll see at birth. This fat layer isn’t just cosmetic. It plays a critical role in helping your baby’s central nervous system regulate body temperature, a skill that’s essential for surviving outside the womb.
During these weeks, babies gain roughly half a pound per week. That pace continues and even accelerates through the final stretch of pregnancy, which is why a baby born at 31 weeks would weigh significantly less than a full-term baby at 39 or 40 weeks, who typically arrives between 6 and 9 pounds.
Why Your Baby’s Weight May Differ
The 3.3-pound average is drawn from population-level data, and individual babies vary for many reasons. Genetics is a major driver. Taller, larger parents tend to have bigger babies, and a baby’s sex plays a role too, with boys and girls following slightly different growth curves. If you’ve had previous pregnancies, your baby may also track differently than a first pregnancy, since first-time mothers tend to carry somewhat smaller babies.
Maternal health conditions shift the picture as well. Diabetes during pregnancy, including gestational diabetes, is associated with larger-than-expected babies because higher blood sugar levels cross the placenta and fuel extra growth. On the other hand, high blood pressure and related conditions like preeclampsia can restrict blood flow through the placenta, slowing fetal growth. The placenta’s position matters too. An anterior placenta (one attached to the front wall of the uterus) can occasionally make it harder to get an accurate size reading on ultrasound.
Nutrition plays a direct role. A well-nourished pregnancy supports steady weight gain, while significant nutritional deficiencies or very low maternal weight gain can result in a smaller baby. Smoking, alcohol, and certain medications also restrict fetal growth.
How Ultrasound Estimates Work
If you’ve been told your baby’s weight at a prenatal visit, that number almost certainly came from an ultrasound. Technicians measure the baby’s head circumference, abdominal circumference, and thigh bone length, then plug those numbers into a formula to estimate weight. It’s the best tool available, but it’s not a scale.
Ultrasound weight estimates carry an accepted margin of error of up to 20% in either direction. For a baby estimated at 3.3 pounds, that means the true weight could reasonably fall anywhere between about 2.6 and 4.0 pounds. This margin is worth keeping in mind if your provider mentions that your baby is measuring “big” or “small.” A single measurement that’s slightly outside the average range often reflects the imprecision of the tool rather than a genuine problem.
What matters more than any single number is the growth trend over time. Your provider tracks whether the baby is following a consistent curve on growth charts, gaining at a steady rate from one visit to the next. A baby who has been in the 30th percentile all along and stays there is following a healthy pattern, even though they’re smaller than average. A baby who drops from the 60th percentile to the 20th over a few weeks is more concerning, because that shift could signal that something is limiting growth.
When Weight Falls Outside the Expected Range
Babies who measure below the 10th percentile for their gestational age are classified as small for gestational age (SGA). At 31 weeks, that would be well under 3 pounds. Some of these babies are simply genetically small and perfectly healthy. Others may not be getting enough nutrients or oxygen through the placenta, a condition called intrauterine growth restriction. When providers suspect this, they’ll typically increase the frequency of monitoring with additional ultrasounds and tests that check blood flow through the umbilical cord.
On the other end, babies above the 90th percentile are considered large for gestational age (LGA). This is more common in pregnancies complicated by diabetes. If your baby is measuring large, your provider may screen for gestational diabetes if they haven’t already, and they’ll monitor growth more closely as your due date approaches.
In both cases, the response at 31 weeks is usually watchful monitoring rather than immediate intervention. There’s still plenty of time for growth patterns to shift, and the goal is to keep the pregnancy going as long as safely possible to give your baby more time to develop.
What Happens in the Weeks Ahead
From 31 weeks on, your baby’s weight will roughly double or even triple before birth. Lungs are maturing, the brain is developing rapidly, and bones are hardening. The fat that’s accumulating now will make up about 15% of your baby’s total body weight by the time they’re born full-term.
If you’re tracking your baby’s growth and feeling anxious about numbers, keep in mind that the range of normal is wide. A healthy 31-week baby could weigh anywhere from about 2.5 to 4 pounds and still be on track. The single most useful thing you can do at this stage is attend your regular prenatal appointments so your provider can watch the growth trend, not just a single snapshot.