At 29 weeks of pregnancy, the average baby weighs about 1,370 grams, or roughly 3 pounds. Most babies at this stage fall somewhere between 2.5 and 3.5 pounds, depending on genetics, maternal health, and how the placenta is functioning. If you’ve just had an ultrasound and want to know whether your baby’s weight looks normal, here’s what the numbers mean.
Average Weight at 29 Weeks
Fetal weight at 29 weeks spans a wide range that’s considered perfectly healthy. The 50th percentile, meaning the midpoint where half of babies weigh more and half weigh less, is about 1,372 grams (just over 3 pounds). Babies at the 10th percentile weigh around 1,177 grams (about 2.6 pounds), and those at the 90th percentile come in near 1,567 grams (roughly 3.5 pounds). All of these weights fall within the normal range.
Crown-to-heel length at this point is approximately 38 to 39 centimeters, or about 15 inches. Your baby is roughly the size of a butternut squash. Over the next several weeks, weight gain accelerates significantly as your baby adds body fat and muscle, so these numbers change quickly.
How Accurate Are Ultrasound Estimates?
It’s worth knowing that estimated fetal weight from an ultrasound is just that: an estimate. The accepted margin of error is up to 20% in either direction, though most estimates land closer to the mark. One large study found the median difference between the projected weight and the actual birth weight was about 6%. So if your ultrasound says your baby weighs 1,300 grams, the true weight could reasonably be anywhere from about 1,220 to 1,380 grams, and potentially further off in some cases.
This means a single measurement that seems slightly high or low isn’t necessarily cause for concern. Your provider will typically track growth over multiple appointments, looking at the trend rather than any one number.
What’s Happening at 29 Weeks
At 29 weeks, your baby is in the middle of a critical growth phase. The brain is developing most rapidly during this period, and your baby can now both see and hear stimuli from outside the womb. Most internal organ systems are well developed, though the lungs are still maturing and would need support if the baby were born early.
Your baby is also building up reserves of body fat, which serve two purposes: regulating body temperature after birth and providing energy. This fat accumulation is a big part of why weight gain picks up so much during the third trimester. You may notice that kicks and jabs feel more like sharp pokes now, since your baby is getting increasingly cramped inside the amniotic sac.
Why Some Babies Measure Smaller
A baby whose estimated weight falls below the 10th percentile for gestational age may be flagged as having fetal growth restriction. At 29 weeks, that threshold is roughly 1,177 grams. Severe growth restriction is defined as weight below the 3rd percentile. Growth restriction diagnosed before 32 weeks is considered early-onset and tends to be monitored more closely because it carries a higher risk of complications.
But a below-average weight doesn’t automatically signal a problem. Many healthy babies simply measure small because their parents are small in stature or because one or both parents were small babies themselves. These are sometimes called “constitutionally small” babies, and they’re growing exactly as expected for their genetics.
When growth restriction does have a medical cause, the most common factors include:
- Maternal health conditions: high blood pressure, autoimmune diseases like lupus, pregestational diabetes, or certain infections
- Placental issues: the umbilical cord attaching in the wrong place on the placenta, or only one umbilical artery forming instead of two, both of which reduce blood flow to the baby
- Fetal factors: heart conditions or genetic differences such as trisomy 21
- Lifestyle factors: smoking, alcohol use, or substance use during pregnancy
- Multiple pregnancies: carrying twins or more means each baby typically weighs less individually
Why Some Babies Measure Larger
A baby above the 90th percentile (roughly 1,567 grams or more at 29 weeks) is considered large for gestational age. The most common reasons are maternal diabetes, including gestational diabetes that develops during pregnancy, and parental genetics. If both parents are tall or were large at birth, a bigger baby is expected. Your provider may monitor blood sugar levels and schedule additional ultrasounds to track growth if your baby consistently measures above the 90th percentile.
What Growth Tracking Looks Like
Rather than fixating on a single weight measurement, providers look at how your baby’s growth tracks over time. A baby who has been consistently around the 25th percentile, for instance, is growing normally for their own curve, even though they’re below average. What raises concern is a baby who was tracking at the 50th percentile and then drops to the 15th, because that shift suggests something may be slowing growth.
If your provider identifies a potential growth issue at 29 weeks, the typical next step is more frequent ultrasounds, often every two to four weeks, along with monitoring blood flow through the umbilical cord. At this stage of pregnancy, there’s still meaningful time for your baby to gain weight before delivery, and most babies who measure slightly small or large at 29 weeks are born at perfectly healthy weights.