How Big Is a Fetus at 5 Months? | Size and Milestones

By the end of the fifth month, a fetus is typically about 10 inches long and weighs around 0.5 to 1.

You might have heard that a 5-month fetus is about the size of an avocado. That comparison gives you a rough visual, but fetal growth at this stage involves a lot more than just length and weight — the numbers change quickly from week to week.

By the end of the fifth month (around week 20), your baby is typically about 10 inches long and weighs between half a pound and a full pound. This article breaks down the measurements, compares them to everyday objects, and walks you through the developmental milestones happening in weeks 17 through 20.

Fetal Size at Month 5: Length and Weight

Doctors measure fetal size two ways. Crown‑rump length (CRL) measures from the top of the head to the bottom of the buttocks — that’s the number used earlier in pregnancy. By month 5, many providers switch to crown‑heel length, which includes the legs and roughly doubles the reported length.

At the start of the fifth month (around week 17), the CRL is about 5 inches. By week 19, the Mayo Clinic notes a CRL of roughly 5½ inches and a weight around 7 ounces. By week 20, the CRL reaches about 6½ inches, per What to Expect.

When you add the legs for head‑to‑heel measurement, the baby reaches about 10 inches by the end of the month. Weight climbs from about 4 ounces at week 17 to 10–16 ounces by week 20 — roughly the difference between an avocado and a small banana.

How Size Comparisons Help (And Why They Vary)

Fruit comparisons are a quick way to picture your baby’s size, but they are rough approximations. Several factors affect how “big” a fetus actually measures at month 5.

• Measurement method: Crown‑rump vs. crown‑heel can double the reported length, so one source’s “5 inches” and another’s “10 inches” may describe the same baby.
• Individual growth: Babies grow at slightly different rates, even within the same week. A week 20 fetus may be 9 inches or 11 inches and still be healthy.
• Due date uncertainty: If your estimated due date is off by a week, the size comparison shifts accordingly.
• Source variation: Different medical sites pick different weeks for their fruit analogy. Cleveland Clinic uses an avocado for month 5; others use a banana for later weeks.

For a more precise picture, your doctor uses ultrasound measurements plotted on a growth chart rather than any single fruit. The comparisons are helpful conversation starters, but they are not diagnostic tools.

What’s Happening in the Fifth Month of Pregnancy

Size isn’t the only story in month 5. As the March of Dimes explains, the baby begins adding a layer of fat under the skin around this time. That fat makes the skin less translucent — it’s no longer see‑through, and the baby starts to look more like a newborn.

Between weeks 17 and 20, the baby’s body also produces a waxy coating called vernix caseosa that protects the skin in the amniotic fluid. Lanugo, a fine hair, appears to help the vernix stick. These changes are part of the gradual shift from a fragile embryo to a baby that could — with intensive medical care — survive if born after week 23.

The rapid growth this month means that by the end of week 20, as WebMD’s fifth month pregnancy size page notes, the baby measures about 10 inches from head to heel and weighs up to 1 pound. That’s roughly a third of the length of a full‑term newborn, but only about one‑tenth of the final weight.

Tracking Your Baby’s Growth Milestones

Each week of month 5 brings a noticeable jump in size and development. Here are the key milestones from the fact doc:

1. Week 17: The baby is roughly 5 inches long (crown‑rump) and weighs about 4 ounces — comparable to an avocado, per the Cleveland Clinic.
2. Week 19: Crown‑rump length reaches about 5½ inches, and weight hits around 7 ounces, according to the Mayo Clinic’s fetal development guide.
3. Week 20: Crown‑rump length is about 6½ inches, and weight is roughly 10 ounces, per What to Expect. Head‑to‑heel length is about 10 inches.

These measurements are averages. Your baby may be a bit smaller or larger, and your provider will track growth trends over multiple checkups rather than relying on a single number.

From Tiny Embryo to Full‑Term Baby

To put the 5‑month size in perspective, consider how much your baby has grown already and how much more growth lies ahead. At 8 weeks, the embryo was a mere 0.63 inches and weighed 0.04 ounces — smaller than a grape. By month 5, the baby has grown to 10 inches and about half a pound.

The remaining weeks focus on adding weight and maturing the lungs, brain, and other organs. The University of Illinois health system’s baby size at 5 months page confirms the same typical range: 10 inches and 0.5 to 1 pound by the end of month five.

From less than an inch at 8 weeks to 10 inches at month 5, your baby’s growth is exponential. The second half of pregnancy is largely about tripling that length and multiplying weight about eightfold.

The Bottom Line

A fetus at 5 months typically measures about 10 inches from head to heel and weighs between half a pound and a full pound. Avocado and banana comparisons can help you visualize, but remember that individual growth varies and measurement methods differ. Your doctor will use ultrasound and fundal height to track your baby’s personal growth curve.

If you have concerns about your baby’s size at a prenatal visit, ask your obstetrician or midwife to walk you through the growth chart. They can explain how your baby compares to typical ranges for your specific due date — and whether any follow‑up is needed.