One of the first questions expectant parents ask at every prenatal visit is simple: "How long is my baby right now?" This baby length calculator turns your gestational age in weeks into an estimated crown-heel length in both centimeters and inches, and pairs it with the fundal height your obstetric provider measures at the belly. Because growth slows and accelerates at different stages, having a ballpark number before the ultrasound makes the appointment easier to follow.
The model is anchored to the two reference points every fetal growth chart shares: at 20 weeks the baby is roughly 25 cm (about 10 inches) long, and by 40 weeks a full-term newborn averages 50 cm (about 20 inches). Drawing a straight line between those anchors gives a clean estimate of about 1.25 cm of length per week, which is close enough for a rough planning tool. Smekys built this page to be fast, private, and free — nothing you type is stored or sent anywhere.
How to Use
- Enter your Gestational Age in completed weeks. This is the number your midwife or doctor calculates from the first day of your last menstrual period (LMP), not from the date you conceived. If an ultrasound re-dated your pregnancy, use that newer figure.
- Set your Average Cycle Length in days. LMP dating assumes a textbook 28-day cycle. If your cycles run longer (say 35 days), ovulation and conception happened about a week later, so the true gestational age is shorter — the calculator subtracts
(cycle − 28) / 7weeks from your LMP figure automatically. - Pick the Baby Sex (if known) option. If your anatomy scan has already shown a boy or a girl, choose that; otherwise leave it on Unknown / not set for the sex-anchored estimate. Boys average about 1 cm longer than girls at term, so the calculator nudges the estimate by +0.5 cm for boys and −0.5 cm for girls.
- Press or tap the calculate button (the result also updates live as you change the number).
- Read the Estimated Baby Length in centimeters and inches — this is the crown-heel length, measured from the top of the head to the heel, which is how babies are measured after 14 weeks.
- Compare the Estimated Fundal Height with the tape measure the clinician wraps around your belly. In a textbook pregnancy it should land within about ±2 cm of the number shown.
- Note the Trimester Code (1 = First, 2 = Second, 3 = Third) so you know which stage of growth the estimate belongs to.
If the number on the result card looks far off from your last ultrasound, trust the ultrasound. Dating by LMP assumes a regular 28-day cycle; people with longer or irregular cycles can easily drift by a week or two.
How Fetal Length Is Measured
Early in pregnancy the baby is curled up, so providers measure the crown-rump length (CRL) — from the top of the head to the bottom of the buttocks. Around 14 weeks the legs uncurled enough that the measurement switches to crown-heel length (CHL), which is the same ruler a newborn gets after birth.
Growth is not perfectly linear. It is fastest in the second trimester and begins to flatten near the end of the third trimester, which is one reason a single straight-line estimate is deliberately approximate. Real variation between healthy babies at the same gestational age can easily reach 10% or more, and boys run roughly 1 cm longer than girls on average at term. These ranges are normal; they are not a diagnosis.
A useful rule of thumb many parents remember: by the middle of pregnancy (about 20 weeks) the baby is about half the length it will be at birth. So if you want a quick mental check, take the term length near 50 cm and halve it — that lands you right back on the 25 cm reference point.
Fundal Height (McDonald's Rule)
Fundal height is the distance from the pubic bone to the top of the uterus, measured in centimeters with a soft tape. Between about 20 and 31 weeks it closely tracks gestational age: the uterus grows roughly 1 cm per week, so a 24-week pregnancy usually shows a fundal height near 24 cm. This is known as McDonald's rule.
After about 36 weeks the baby usually drops lower into the pelvis (lightening), and fundal height readings can soften or even dip slightly, so the calculator's fundal figure is most reliable in the middle window. A measurement more than 2 cm above or below the expected number prompts providers to check the dating, the amount of amniotic fluid, and the baby's size — again, this is a screening signal, not an answer by itself.
Examples
Example 1 — At 20 weeks. Enter 20 weeks. If the baby is unknown sex, the estimate is 25 cm (9.84 in) long, with a fundal height near 20 cm (7.87 in). If the anatomy scan showed a boy, the estimate nudges up to 25.5 cm (10.04 in); for a girl it nudges down to 24.5 cm (9.65 in). This is the classic moment the uterus reaches the level of the belly button, and most parents are amazed to learn their baby is already about ten inches long.
Example 2 — At 30 weeks. Enter 30 weeks. With unknown sex, length estimates to 37.5 cm (14.76 in) and the fundal height to about 30 cm (11.81 in). With a boy it reads 38 cm (14.96 in) and with a girl 37 cm (14.57 in). Growth is still strong, and the baby now fills a large share of the uterus — around the 50th percentile for a typical third-trimester size.
Example 3 — At 40 weeks. Enter 40 weeks. With unknown sex the estimate is 50 cm (19.69 in) and the fundal height about 40 cm (15.75 in). With a boy it reads 50.5 cm (19.88 in) and with a girl 49.5 cm (19.49 in). This matches the average newborn length of roughly 50 cm; healthy full-term babies commonly range from about 46 cm to 52 cm, a spread of nearly 12% around the average.
Example 4 — Long cycles at 20 weeks LMP. Suppose your LMP dating says 20 weeks but your cycles typically run 35 days. With unknown sex, the calculator adjusts down to an estimated 19 weeks of true gestational age and reports a length of 23.75 cm (9.35 in) instead of 25 cm. This is the same reason an ultrasound often "re-dates" pregnancies with irregular or long cycles.
FAQ
How is a baby's length estimated by week?
The estimate uses two well-known reference points — about 25 cm at 20 weeks and 50 cm at 40 weeks — and draws a straight line between them, giving roughly 1.25 cm per week. The result is converted to inches by dividing centimeters by 2.54. It is a planning estimate, not a substitute for an ultrasound.
What is the difference between crown-rump and crown-heel length?
Crown-rump length (CRL) measures head to rump and is used in the first trimester when the legs are tucked in. Crown-heel length (CHL) measures head to heel and is used from about 14 weeks onward, matching the way newborns are measured after birth.
How accurate is this baby length calculator?
It is approximate on purpose. Healthy babies at the same gestational age can differ by 10% or more, and LMP dating can be off by a week or two if your cycles are irregular. For exact measurements, a second-trimester ultrasound is the gold standard.
What should my fundal height be at 20 weeks?
By McDonald's rule, fundal height in centimeters should be close to your weeks of pregnancy, so around 20 cm (about 7.9 in) at 20 weeks, give or take 2 cm. It is most reliable between 20 and 31 weeks.
Why does fundal height stop tracking so well near 40 weeks?
Around 36 weeks the baby often drops head-first into the pelvis, which lowers the top of the uterus and can make fundal height readings flatten or dip. This is normal and is why the calculator's fundal estimate is framed as a middle-pregnancy guide.
Does the baby's length differ between boys and girls?
Slightly. At term, boys average about 1 cm longer than girls, but the overlap is large — plenty of girls are longer than average boys. Sex-based differences are small compared with the overall normal range.
Why does my cycle length change the estimated baby length?
LMP dating assumes a textbook 28-day cycle where ovulation happens on day 14. If your cycles run longer — say 35 days — ovulation and conception actually happen about a week later, so your true gestational age is shorter than the calendar count from your last period. Subtracting (cycle − 28) / 7 weeks re-aligns the estimate with the actual time since conception.
Disclaimer
This baby length calculator is provided for informational purposes only and is not medical advice. Estimates are based on your entered gestational age and a simplified linear growth model; actual measurements vary widely between individuals and between pregnancies. Fundal height and length are screening references only. Always confirm your baby's growth, dating, and fundal measurements directly with a qualified obstetrician, midwife, or other prenatal care professional, and seek care promptly for any concerning symptoms.