Fetal Weight Calculator
See the average weight and length for each week of pregnancy.
Enter the first day of your last period and we'll show the average fetal weight and length for your current week, plus the full week-by-week chart from week 8, when there's enough there to measure, through week 42. These are population averages, not a measurement of your baby specifically, an ultrasound is the only way to get that.
Weights and lengths are averages from standard fetal growth references, individual babies vary quite a bit and still fall within a normal range. Length is measured crown to rump (head to bottom) through about week 20, when the legs are still curled up, then head to heel afterward, which is why the length jumps between those weeks rather than climbing smoothly.
The number this Fetal Weight Calculator gives you is an average, not a prediction about your specific baby. Here’s where that average actually comes from and why your baby doesn’t need to match it exactly to be perfectly healthy. 
Where These Numbers Come From
The weights and lengths behind this calculator come from published fetal growth references, the same kind ACOG walks through in How Your Fetus Grows During Pregnancy, built from large numbers of pregnancies rather than any one baby. They’re meant to describe what’s typical at each week, the same way a pediatric growth chart describes typical height and weight for a toddler’s age.
Why Two Babies At The Same Week Can Look So Different
What This Calculator Can’t Tell You