Your baby starts pregnancy smaller than a poppy seed and grows to roughly 7.5 pounds and 20 inches by birth. How big your baby is right now depends on how far along you are, and size can vary quite a bit from one pregnancy to another. Here’s a detailed look at what to expect from the first weeks through delivery.
Week-by-Week Size in the First Trimester
Growth in the first trimester is dramatic in relative terms. Your baby doubles and triples in size repeatedly, even though the actual measurements are tiny. During this stage, size is measured from the top of the head to the bottom of the torso (not including the legs), because your baby is curled up tightly.
- Week 4: Poppy seed
- Week 5: Sesame seed
- Week 6: Lentil
- Week 7: Blueberry
- Week 8: Raspberry
- Week 9: Grape
- Week 10: Strawberry
- Week 11: Fig
- Week 12: Lime
- Week 13: Plum
At your first ultrasound (typically between weeks 9 and 14), your provider measures from crown to rump. That measurement, usually between 30 and 84 millimeters at this stage, is one of the most accurate ways to confirm your due date.
Second Trimester: Rapid Lengthening
Around week 14, measurements shift to include full body length from head to heel, because your baby begins to stretch out. This is the trimester when your baby goes from the size of a lemon to roughly the length of an ear of corn. Organs mature, bones harden, and your baby starts putting on a thin layer of fat.
- Week 14: Lemon
- Week 15: Apple
- Week 16: Avocado
- Week 17: Turnip
- Week 18: Bell pepper
- Week 19: Pomegranate
- Week 20: Banana (length)
- Week 21: Mango
- Week 22: Sweet potato
- Week 23: Grapefruit
- Week 24: Ear of corn
- Week 25: Acorn squash
- Week 26: Spaghetti squash
- Week 27: Head of cauliflower
Starting around week 20, your provider will likely measure your belly at each visit. The distance from your pubic bone to the top of your uterus, measured in centimeters, should roughly equal your week of pregnancy, plus or minus 2 centimeters. At 24 weeks, for example, a measurement between 22 and 26 centimeters is normal. This simple check helps track growth between ultrasounds.
Third Trimester: The Big Weight Gain
The third trimester is when your baby really fills out. Length growth slows down, but weight gain accelerates. Starting around week 35, your baby enters the most rapid weight-gain period of pregnancy, packing on roughly 8 to 12 ounces (about half to three-quarters of a pound) every single week.
- Week 28: Large eggplant
- Week 29: Butternut squash
- Week 30: Large cabbage
- Week 31: Coconut
- Week 32: Papaya
- Week 33: Pineapple
- Week 34: Cantaloupe
- Week 35: Honeydew melon
- Week 36: Head of romaine lettuce
- Week 37: Bunch of Swiss chard
- Week 38: Mini watermelon
- Week 39: Pumpkin
- Week 40: Watermelon
By the time you reach full term, the average baby weighs about 3.5 kg (7.5 pounds) and measures around 50 cm (20 inches) long. Anything between 5.5 and 10 pounds is considered within the normal range, and length typically falls between 18 and 22 inches.
What Growth Percentiles Mean
Your ultrasound report or prenatal visit may mention a growth percentile. This number tells you how your baby’s estimated weight compares to other babies at the same gestational age. A baby at the 50th percentile is right in the middle. A baby at the 10th percentile is smaller than 90% of babies at that stage, and a baby at the 90th percentile is larger than 90%.
To put real numbers on it: at 40 weeks, a boy at the 10th percentile weighs about 6 pounds 12 ounces, while a boy at the 90th percentile weighs about 9 pounds 4 ounces. For girls, the range runs from roughly 6 pounds 8 ounces at the 10th percentile to about 8 pounds 14 ounces at the 90th. That’s a spread of more than 2 pounds between the smaller and larger end of normal.
Babies who consistently track along their own curve, even at the 15th or 85th percentile, are generally growing well. What providers watch for is a sudden jump or drop across percentile lines, which can signal a change worth investigating.
Why Some Babies Are Bigger or Smaller
Several factors influence how big your baby grows. Genetics play a major role on both sides: your height, your partner’s height, and your own birth weight all contribute. Babies also tend to be slightly larger in subsequent pregnancies compared to first pregnancies, and boys tend to weigh a bit more than girls at the same gestational age.
Maternal health conditions matter too. Blood sugar levels during pregnancy have a direct effect on fetal growth. Even mildly elevated blood sugar that doesn’t meet the threshold for gestational diabetes can nudge a baby toward the higher end of the weight range. Blood pressure, nutrition, vitamin D levels, and stress hormones like cortisol also play a role. The placenta acts as the middleman for all of this, so its health and efficiency influence how much nourishment actually reaches your baby.
If your baby is estimated to weigh more than about 9 pounds 15 ounces (in someone with diabetes) or more than 11 pounds (without diabetes), that’s considered macrosomia, a clinical term for an unusually large baby. This can affect delivery planning, but it’s relatively uncommon.
How Accurate Are Size Estimates
Ultrasound weight estimates are helpful but imperfect, especially later in pregnancy. At term (37 weeks and beyond), about 73% of ultrasound weight estimates fall within 10% of the baby’s actual birth weight. That means if your baby is estimated at 8 pounds, the true weight is likely somewhere between 7 pounds 3 ounces and 8 pounds 13 ounces. About 96% of estimates land within 20% of the real number.
The less precise estimates tend to happen at the extremes: very large and very small babies are harder to measure accurately. So if you’re told your baby is measuring big or small, keep that margin of error in mind. Many parents have been told to expect a 9-pound baby and delivered one closer to 7.5 pounds, or vice versa.
The belly measurement your provider takes at each visit (fundal height) is even more of a rough guide. It works best between weeks 20 and 36. Before 20 weeks, the uterus hasn’t risen high enough to measure reliably. After 36 weeks, the baby may drop lower into your pelvis, making the measurement less predictive. Factors like your body type, the baby’s position, and the amount of amniotic fluid all influence the number.
What Matters More Than a Single Number
Your baby’s size at any one moment is less important than the overall growth trend. A baby who measures at the 25th percentile at 28 weeks and stays near the 25th percentile at 36 weeks is following a healthy, consistent pattern. A baby who drops from the 50th to the 10th percentile over a few weeks is the scenario that warrants a closer look, typically with more frequent ultrasounds to monitor blood flow and fluid levels.
Growth also isn’t perfectly steady. Babies have spurts and slower stretches, just like children do after birth. A single measurement that seems off isn’t automatically a concern, which is why providers look at the full picture across multiple visits rather than reacting to one data point.