A fetus grows from the size of a poppy seed at week 4 to roughly the size of a small pumpkin by week 40, gaining most of its weight in the final weeks of pregnancy. At the earliest stages, growth is measured in millimeters. By birth, the average baby is about 19 to 21 inches long and weighs around 7.5 pounds.
First Trimester: Rapid Change in Tiny Increments
For the first eight weeks, the developing baby is technically called an embryo. At week 4, it’s about the size of a poppy seed. By week 7, it’s roughly a blueberry. Growth accelerates quickly from here: at week 8, the crown-to-rump measurement (the distance from the top of the head to the bottom of the torso) is about 16 millimeters, or just over half an inch. By the end of week 10, that measurement reaches around 41 millimeters, close to 1.6 inches, comparable to a kumquat.
At week 9, the embryo officially becomes a fetus. At this point, the head makes up about half the total length, which is why early images on ultrasound can look top-heavy. By week 12, the crown-to-rump length hits roughly 54 to 65 millimeters (about 2 to 2.5 inches), around the size of a lime. By week 13 to 14, measurements reach 67 to 85 millimeters (roughly 3 inches), and all major organ systems are in place even though the fetus weighs only about an ounce.
How Fetal Size Is Measured
During the first trimester, ultrasounds measure crown-rump length, the straight-line distance from the top of the head to the base of the spine. This is the most accurate sizing tool early on because the fetus is curled up and the legs are too small and flexed to measure reliably.
After about 12 weeks, providers switch to a set of individual measurements: the head circumference, the distance between the two sides of the skull, the abdominal circumference, and the femur (thigh bone) length. Software plugs these numbers into formulas to estimate overall size and weight. These later estimates are useful but less precise. Ultrasound-based weight estimates carry an accepted margin of error of up to 20% in either direction, which means a fetus estimated at 7 pounds could actually weigh anywhere from about 5.5 to 8.5 pounds. This is one reason your provider may give you a range rather than a single number.
Second Trimester: Length Before Weight
The second trimester is when the fetus stretches out dramatically. At week 15, it’s about the size of an apple, roughly 4 inches from crown to rump. By week 19, it’s comparable to an heirloom tomato, and by week 23, a large mango. During this period, most of the growth shows up in length rather than weight. The fetus goes from about 4 inches to nearly 12 inches (measured head to heel) between weeks 14 and 27, but it only weighs around 1.5 to 2 pounds by the end of the second trimester.
This is also the trimester when the anatomy scan typically happens, around weeks 18 to 22. That ultrasound provides the most detailed look at the baby’s measurements and gives your provider a benchmark for tracking growth in the months ahead. The fetus is still lean at this stage, with very little body fat, and its skin appears translucent.
Third Trimester: The Weight Gain Sprint
The final 12 weeks are all about filling out. At week 28, the fetus is about the size of a large eggplant and weighs roughly 2 to 2.5 pounds. By week 31, it’s comparable to a coconut. Then, starting around week 35, the most rapid weight gain begins: the fetus puts on about 8 to 12 ounces per week, or roughly half a pound to three-quarters of a pound every seven days. At week 35, it’s around the size of a honeydew melon.
This late-stage weight gain is mostly body fat, which the baby needs for temperature regulation after birth. In the final five weeks alone, a fetus can gain 3 to 4 pounds. By week 40, the average newborn measures 19 to 21 inches from head to toe and weighs between 6 and 9 pounds, with 7.5 pounds being a common average. That’s roughly the size of a small pumpkin.
Week-by-Week Size Comparisons
Fruit and vegetable comparisons are imperfect, but they give you a quick mental image of what’s happening inside:
- Week 4: Poppy seed
- Week 7: Blueberry
- Week 10: Kumquat
- Week 12: Lime
- Week 15: Apple
- Week 19: Heirloom tomato
- Week 23: Large mango
- Week 25: Rutabaga
- Week 28: Large eggplant
- Week 31: Coconut
- Week 35: Honeydew melon
- Week 40: Small pumpkin
These comparisons capture overall volume, not just length or weight. A 28-week fetus and a large eggplant are similar in length, but the eggplant is heavier. Still, the analogies are useful for visualizing how much space the baby takes up at each stage.
What Affects Fetal Size
Not every fetus follows the same growth curve. Genetics play the biggest role: taller parents tend to have longer babies, and birth weight often runs in families. The birth parent’s nutrition and overall health also matter. Conditions like gestational diabetes can lead to larger-than-average babies, while high blood pressure or placental problems can restrict growth.
First babies tend to be slightly smaller than subsequent babies from the same parent. Carrying multiples also changes the equation. Twins and triplets typically weigh less individually than singletons at the same gestational age because they’re sharing space and nutrients.
Your provider tracks fetal growth at regular prenatal visits, and if measurements fall significantly above or below the expected range, they’ll usually order more frequent ultrasounds to monitor the trend. A single measurement that’s a bit high or low is rarely a concern on its own. What matters more is whether growth is following a consistent curve over time.