Your baby’s size changes dramatically from week to week, starting smaller than a poppy seed and reaching roughly 20 inches long by birth. How big your baby is right now depends on how far along you are, and the easiest way to picture it is by comparing your baby to familiar fruits and vegetables at each stage.
Week-by-Week Size Comparisons
During the first trimester, growth happens fast but on a tiny scale. At week 4, your baby is about the size of a poppy seed. By week 7, it’s a blueberry. At week 10, think of a kumquat, and by week 12, your baby is roughly the size of a lime.
The second trimester is when things start to feel more real. At week 15, your baby is about the size of an apple. By week 19, it’s comparable to an heirloom tomato, and at week 23, a large mango. Week 25 puts your baby at roughly the size of a rutabaga.
In the third trimester, growth accelerates significantly. At week 28, your baby is comparable to a large eggplant. By week 31, think of a coconut. Week 35 brings your baby to about the size of a honeydew melon, and at week 40 (full term), your baby is the size of a small pumpkin, averaging around 20 inches long for boys and 19¾ inches for girls.
How Your Baby Is Actually Measured
The fruit comparisons are fun, but your doctor tracks your baby’s size with much more precise ultrasound measurements. What gets measured changes as your pregnancy progresses.
From about 6 to 13 weeks, your care provider measures crown-rump length, which is the distance from the top of your baby’s head to the bottom of the torso. At this stage, your baby’s legs are so small and curled up that a head-to-toe measurement wouldn’t be meaningful. This early measurement is one of the most accurate ways to date your pregnancy.
After 13 weeks, ultrasound shifts to a set of measurements called fetal biometry. Instead of one simple length, your provider looks at several numbers: the diameter across the head, the circumference around the head, the circumference around the belly, and the length of the thighbone. Together, these give a much more complete picture of how your baby is growing than any single number could. The head-to-heel lengths you see in pregnancy apps are estimates based on these measurements, not something that gets directly measured on an ultrasound.
What “Measuring Big” or “Measuring Small” Means
At some point during pregnancy, many parents hear that their baby is measuring larger or smaller than expected. This can trigger worry, but it helps to understand the numbers behind these labels.
Doctors compare your baby’s measurements against growth charts for the same gestational age. Most providers aren’t concerned unless a baby falls below the 10th percentile or above the 90th percentile. A baby below the 10th percentile may be classified as small for gestational age, while one above the 90th percentile is considered large for gestational age. At full term, a birthweight over about 8 pounds 13 ounces (4,000 grams) is the threshold for macrosomia, the clinical term for a particularly large baby.
Being outside the typical range doesn’t automatically mean something is wrong. Some babies are simply genetically small or large, just like adults come in different sizes. But it does signal your provider to look more closely. Possible causes include issues with the placenta, the birthing parent’s health conditions like diabetes or high blood pressure, or occasionally a developmental concern with the baby. In most cases, additional monitoring is all that’s needed.
Why Size Estimates Can Be Off
Ultrasound size estimates in the third trimester carry a margin of error that surprises many parents. Weight predictions based on ultrasound can be off by as much as 15% in either direction. That means a baby estimated at 7 pounds could realistically weigh anywhere from about 6 to 8 pounds at birth. This is partly why a baby predicted to be “really big” sometimes arrives at a perfectly average weight, or vice versa.
The accuracy of measurements also depends on your baby’s position, the amount of amniotic fluid, and how far along you are. Earlier measurements tend to be more reliable for dating, while later ones are better at tracking growth trends over time rather than pinpointing an exact size. If your provider mentions your baby is measuring a little ahead or behind, the trend across multiple appointments matters more than any single number.
What Affects Your Baby’s Size
Genetics plays the largest role. Taller parents tend to have longer babies, and birthweight patterns often run in families. Beyond genetics, several factors influence how big your baby grows. Nutrition during pregnancy matters, particularly getting enough protein and calories in the second and third trimesters when your baby is gaining the most weight. Babies tend to gain about half a pound per week in the final weeks of pregnancy, so those last few weeks make a noticeable difference in birth size.
First babies tend to be slightly smaller than subsequent ones. Carrying multiples typically means each baby will be smaller than a singleton at the same gestational age. And conditions like gestational diabetes can cause a baby to grow larger than average, because extra glucose crossing the placenta gets stored as fat. On the flip side, high blood pressure or placental problems can restrict blood flow and nutrients, leading to a smaller baby.