Baby Size by Week Calculator
See how big your baby is each week of pregnancy with fruit size comparisons. Enter values for instant results with step-by-step formulas.
Formula
Growth data based on WHO and ACOG median percentile charts
Length and weight values represent the 50th percentile from large-scale clinical studies. Crown-to-rump measurement is used for weeks 4-19, and crown-to-heel measurement from week 20 onward.
Worked Examples
Example 1: Week 20 Mid-Pregnancy Check
Problem: A mother wants to know her baby's size at 20 weeks of pregnancy.
Solution: At week 20, the baby measures approximately 25.6 cm (10.1 inches) crown-to-heel and weighs about 300 grams (10.6 oz). The baby is comparable in size to a banana. This marks the halfway point of pregnancy, with the mother 50% through gestation and approximately 140 days remaining until the due date.
Result: Length: 25.6 cm (10.1 in) | Weight: 300g (10.6 oz) | Size: Banana | 50% complete
Example 2: Week 32 Third Trimester Growth
Problem: A mother at 32 weeks wants to track her baby's growth progress.
Solution: At week 32, the baby measures approximately 42.4 cm (16.7 inches) crown-to-heel and weighs about 1,702 grams (3.75 lbs). The baby is roughly the size of a jicama. The mother is in the third trimester with 80% of the pregnancy complete and about 56 days (8 weeks) remaining until the estimated due date.
Result: Length: 42.4 cm (16.7 in) | Weight: 1,702g (3.75 lbs) | Size: Jicama | 80% complete
Frequently Asked Questions
How accurate are baby size estimates during pregnancy?
Baby size estimates are based on population averages gathered from ultrasound measurements of thousands of pregnancies. Individual babies can vary significantly from these averages, sometimes by as much as 15 to 20 percent in either direction. Factors like genetics, maternal nutrition, and placental function all influence fetal growth. Your doctor uses ultrasound measurements of head circumference, abdominal circumference, and femur length to create a more personalized estimate for your specific baby. These charts provide a helpful general guideline, but the ultrasound measurements taken by your healthcare provider are always more accurate for your individual pregnancy.
Why do baby size measurements change from crown-to-rump to crown-to-heel?
Before week 20, babies are measured from crown to rump (the top of the head to the bottom of the buttocks) because their legs are curled up tightly against their torso, making it nearly impossible to get an accurate full-length measurement. After week 20, the baby has grown large enough and their legs have straightened sufficiently to allow a reliable crown-to-heel measurement. This transition means there is a noticeable jump in the reported length around week 20, which can be confusing for parents. The change in measurement technique is standard medical practice used worldwide and does not indicate a sudden growth spurt. Understanding this distinction helps parents track growth more accurately throughout the pregnancy.
Is my baby too small or too big for their gestational age?
Babies naturally come in a wide range of sizes, and the numbers shown here represent averages from large population studies. A baby is generally considered small for gestational age (SGA) if they fall below the 10th percentile in weight, and large for gestational age (LGA) if above the 90th percentile. Your healthcare provider will track your baby's growth over multiple appointments to identify trends rather than relying on a single measurement. Consistent growth along a particular percentile curve is typically more important than the exact percentile itself. If your provider has concerns about your baby's growth rate, they may order additional ultrasounds or monitoring to ensure everything is progressing well.
How much weight does a baby gain during the third trimester?
The third trimester is a period of rapid weight gain for the developing baby. At the start of week 28, the average baby weighs roughly 1,000 grams or about 2.2 pounds. By week 40, the average weight has increased to approximately 3,400 to 3,500 grams, which is about 7.5 pounds. This means the baby roughly triples its weight during the final 12 weeks of pregnancy. During the last few weeks, a baby typically gains about 200 to 250 grams per week, or roughly half a pound. This rapid growth is why maternal caloric needs increase during the third trimester, and why the mother often feels significantly more physical discomfort as the baby grows.
How does Baby Size by Week Calculator determine the weight and length for each week?
Baby Size by Week Calculator uses data compiled from multiple large-scale medical studies and clinical growth charts, including those published by the World Health Organization and the American College of Obstetricians and Gynecologists. The values represent the 50th percentile, meaning the median or average size for babies at each gestational week. These reference values are derived from ultrasound measurements of thousands of pregnancies across diverse populations. The length measurements use crown-to-rump for weeks prior to 20 and crown-to-heel measurements afterward, following standard obstetric practice. While these averages provide a reliable benchmark, individual variation is completely normal and expected.
Can I use Baby Size by Week Calculator on a mobile device?
Yes. All calculators on NovaCalculator are fully responsive and work on smartphones, tablets, and desktops. The layout adapts automatically to your screen size.