Baby Size by Week Calculator

The figures shown are median reference values, not a formula applied to your pregnancy. Each gestational week maps to an approximate 50th percentile length and weight drawn from fetal biometry, and the shape of that curve is worth understanding. Growth in length is fastest in relative terms during the first trimester, while growth in mass is concentrated late: the fetus is around 300 g at 20 weeks, roughly 1,000 g at 28 weeks, and about 3,400 g at 40 weeks, gaining on the order of 200 g per week through the final month. The apparent jump at week 20 is a change of measurement convention, not a growth spurt. Before 20 weeks the fetus is measured crown to rump because the legs are flexed against the torso, and from 20 weeks onward the reference switches to crown to heel, which adds the leg length in one step. Gestational age here is dated from the first day of the last menstrual period, the standard clinical convention, and is therefore about two weeks longer than the actual post-conception age. Individual variation is wide and normal: the 10th and 90th percentile weights at a given week can differ by well over a kilogram near term. A fetus below the 10th percentile is classed as small for gestational age and above the 90th as large for gestational age, but clinicians weigh the trajectory across serial scans more heavily than a single point, since consistent growth along any percentile is generally reassuring. Sonographic weight estimation itself carries an error of roughly 10 to 15 percent, so ultrasound reports and these medians will rarely agree exactly. The fruit comparisons are a media convention with no clinical standing and differ between sources.

Fetal size was inaccessible before imaging; obstetricians estimated growth by palpation and by symphysis-fundal height, a tape measurement from the pubic bone to the top of the uterus that remains in use as a screening tool. Diagnostic ultrasound changed that. Ian Donald, working with the engineer Tom Brown and the obstetrician John MacVicar in Glasgow, adapted industrial flaw-detection equipment to obstetric use and published their landmark Lancet paper on investigating abdominal masses by pulsed ultrasound in 1958, opening the way to direct measurement of the fetus in utero. Quantitative biometry followed over the next three decades. Crown-rump length was established as the most accurate early dating measurement, with Hugh Robinson and J. E. E. Fleming publishing a widely adopted crown-rump length dating chart in 1975. For later pregnancy, Frank Hadlock and colleagues at Baylor College of Medicine published a series of regression equations in the early to mid 1980s estimating fetal weight from combinations of biparietal diameter, head circumference, abdominal circumference, and femur length; the Hadlock formulas remain the default in most ultrasound machines today. Because early charts were derived from small, geographically narrow populations, later projects sought prescriptive international references: the INTERGROWTH-21st Project published fetal growth standards in 2014 from healthy, well-nourished cohorts across eight countries, and the World Health Organization released its own multinational fetal growth charts in 2017. Debate continues over whether such universal standards or population-customized charts better identify pathological growth restriction, which is why reference tables from different sources do not fully agree.