What this calculator does
See how big a typical baby is at your week of pregnancy. Enter your week, or let the calculator work it out from your last period or due date. You get the typical weight and length, your trimester, and how much weight a typical baby gains each week.
These numbers are averages for the week. They are not a measurement of your baby, and babies vary a lot. Two healthy babies of the same age can differ by hundreds of grams. Only an ultrasound by your healthcare provider measures your own baby.
Where it's used
Why this math matters outside the classroom:
- Counting weeks. Expecting parents find that 90 days after a period started is 12 weeks 6 days, since 90 ÷ 7 is 12 with 6 left over.
- Grams to ounces. A typical 28-week weight of 1,210 g is about 42.7 oz, or 2 lb 10.7 oz, after dividing by 28.35.
- Weekly gain. Students subtract weekly averages to see growth speed up: 15 g in week 13, but 169 g in week 29.
- Planning ahead. Families use the due date and the weeks left to plan leave, a car seat and childcare start dates.
How to use it
- Choose how to start: the week you're in, the first day of your last period, or your due date.
- Enter the week (10 to 40) or the date. For the date options, leave "Counting from" as today, or change it to check another day.
- Read the typical weight in grams, with pounds and ounces beside it. Below it are the typical length, how far along you are, and the weekly gain.
- Open the table to see every week from 10 to 40 in one chart.
What the inputs mean
- Week of pregnancy
- Completed weeks since the first day of your last period. For example, 20 weeks 4 days counts as week 20.
- First day of last period
- The first day of bleeding of your most recent period, not the last day. The calculator assumes a 28-day cycle.
- Estimated due date
- The date your healthcare provider gave you, or one from a due date calculator.
- Counting from
- The day to work out the week for. It starts as today.
Formula and method
Gestational age = counting-from date − first day of last period, in weeks + days From a due date: first day of last period = due date − 280 days Typical weight (g) = e^(0.578 + 0.332 × W − 0.00354 × W²) Weight in ounces = grams ÷ 28.349523125 Early length (mm) = ((W × 7 − 23.73) ÷ 8.052)² Time left = 280 days − days so far
W is the number of completed weeks of pregnancy. Gestational age is how far along a pregnancy is. It is counted from the first day of the last period, the way doctors count it. The letter e is a fixed number, about 2.718.
The weight formula comes from Hadlock and colleagues (1991). They built it from ultrasound weight estimates and gave it for 10 to 40 weeks. It gives the middle, or 50th percentile, weight: half of babies weigh more than this and half weigh less.
Length is measured in two ways. Early on, babies are measured from the top of the head to the bottom. This is called crown-rump length. The calculator uses the Robinson and Fleming (1975) ultrasound formula for weeks 10 to 13. After about 14 weeks, the legs are long and curled, so an ultrasound measures parts of the body instead.
Later lengths are crown-heel length: head to heel, with the legs straight. That can only be measured after birth. So weeks 24 to 40 use the INTERGROWTH-21st study of babies measured at birth. The calculator takes the average of its figures for boys and girls. For weeks 14 to 23, we found no reliable published average, so no length is shown.
The everyday comparison picks the item closest in weight. It compares by ratio, so being 40 grams off a 50-gram item counts as much as 400 grams off a 500-gram one.
Worked example
The first day of the last period was January 1, 2026. How big is a typical baby on April 1, 2026?
- Days so far: Apr 1, 2026 − Jan 1, 2026 = 90 days.
- 90 ÷ 7 = 12 weeks, 6 days. That's 12 completed weeks, so week 12 is used.
- Weight: 0.578 + 0.332 × 12 − 0.00354 × 12² = 4.0522, and e^4.0522 = 58 g. That's 58 ÷ 28.35 = 2.0 oz.
- Length: ((84 − 23.73) ÷ 8.052)² = 56 mm, or 5.6 cm (2.2 in) from crown to rump.
- Weekly gain: 58 − 45 = 13 g since week 11, and 73 − 58 = 15 g by week 13.
- Comparison: 58 ÷ 67 = 0.87, so about 87% of the weight of a lime.
- Due date: Jan 1, 2026 + 280 days = Thursday, Oct 8, 2026. Time left: 280 − 90 = 190 days, or 27 weeks, 1 day.
Result: At 12 weeks, 6 days (first trimester), a typical baby weighs about 58 g (2.0 oz) and measures about 5.6 cm from head to bottom.
Understanding the result
Think of these numbers as the middle of a wide range. A baby well above or below them can be perfectly healthy. Genes, the parents' size and many other things play a part.
Watch the jump in length at week 24. The baby didn't suddenly grow. The measure changes from head-to-bottom to head-to-heel, which adds the legs.
If you've had a scan, your report may show an estimated fetal weight. Only your healthcare provider can say what it means for your baby. This page can't tell you whether a baby is growing well.
Assumptions and limitations
- The week is counted from the first day of the last period, with a 28-day cycle and a due date 280 days after that day.
- Sizes are given for whole weeks. A baby at 20 weeks 6 days is shown with the 20-week figures.
- From 41 weeks 0 days to 42 weeks 6 days, the 40-week figures are shown, since the weight formula stops at 40 weeks.
- These are averages for a population, not a measurement or a prediction for any one baby. Twins and other multiples are often smaller.
- The weight standard starts at 10 weeks, so earlier weeks aren't covered.
- No length is shown for weeks 14 to 23, because we found no reliable published average to use.
- Later lengths come from babies measured at birth. Babies born early may differ a little from babies of the same age still in the womb.
- It is not a medical tool. It can't detect any problem, and it doesn't replace prenatal care.
Frequently asked questions
Why does the length jump at 24 weeks?
The kind of length changes. Up to 13 weeks, babies are measured from head to bottom. From 24 weeks, the figures are head to heel, with the legs stretched out. Legs add a lot of length, so the numbers jump even though growth is smooth.
Is my baby too big or too small if my scan shows a different weight?
This page can't tell you that. The weight here is an average, and healthy babies vary widely around it. Ultrasound weights are also estimates. Your healthcare provider looks at your scans and your history together, so ask them about your baby's growth.
Why is there no length for weeks 14 to 23?
By then, an ultrasound measures the head, belly and thigh bone rather than the whole body. Head-to-heel length can only be measured after birth. We found no reliable published average for these weeks that we could check, so we leave them blank rather than guess.
Why does the calculator start at 10 weeks?
The weight standard used here starts at 10 weeks. Before that, a baby weighs only a few grams, and published averages are hard to find.
Are the dates I enter stored or shared?
No. The math runs in your browser, and this calculator doesn't create a shareable link with your dates.
How this calculator is checked
Method: Weight: Hadlock, Harrist & Martinez-Poyer, Radiology 1991;181:129–33, 50th-percentile equation ln(EFW) = 0.578 + 0.332·GA − 0.00354·GA² (10–40 weeks). Crown-rump length, weeks 10–13: Robinson & Fleming, Br J Obstet Gynaecol 1975;82:702–10, GA(days) = 8.052·√CRL + 23.73, inverted. Crown-heel length, weeks 24–40: INTERGROWTH-21st length at birth, 50th centile, mean of boys and girls (Villar et al., Lancet 2014;384:857–68 and Lancet 2016;387:844–5). Gestational age from LMP, due date = LMP + 280 days. Sources and checks are recorded in src/lib/health/fetal-size.ts. The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-26. Found a problem? Tell us.
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