Fetus Size 18 Weeks | What Baby Growth Looks Like

Reaching 18 weeks often means a detailed scan and a long list of numbers on the report. It is easy to stare at the measurements and wonder whether baby is too small, too big, or right on track.

In the middle of the second trimester, growth is fast but also widely varied. Charts describe averages, not a single “correct” size, and different guides often use different ways to measure length. Knowing what those figures actually mean can take a lot of pressure out of reading an ultrasound report.

This article walks through typical baby size at 18 weeks, how professionals measure growth, why popular charts sometimes disagree, and when it makes sense to contact your own team for personal advice. It offers general information only and does not replace care from your midwife or doctor.

Baby Size At 18 Weeks Of Pregnancy: Length And Weight

Around 18 weeks, many national health services describe baby as roughly 14 centimetres from head to bottom and close to 190 grams in weight, about the size of a bell pepper. The NHS week 18 pregnancy guide gives a head-to-bottom length of 14.2 cm and compares baby to a pepper in your hand.

Growth charts that list a longer length, sometimes around 22 centimetres at 18 weeks, are usually measuring from crown to heel instead of crown to rump. Fetal development references built from large ultrasound studies list an average 18-week length close to 22.2 cm and a weight just above 220 grams, which fits well with those longer measurements.

Average Length And Weight At 18 Weeks

Two main length measures appear in pregnancy charts:

• Crown-rump length (CRL): head to bottom. This is standard up to about 20 weeks, while baby’s legs are tucked up.
• Crown-heel length (CHL): head to heel. This gives a taller number and is common in general growth tables.

When one chart lists a CRL of about 14 cm and another lists a CHL around 22 cm, both can describe the same stage of pregnancy. They are simply using different endpoints on the body. In both cases, a weight around 190–220 grams is typical, with some babies a bit smaller and some already above that band.

Many reputable charts, including those that draw on ultrasound-based fetal growth standards, show that healthy babies at 18 weeks can sit below or above the midline while still following a steady curve.

18 Week Fetal Size Averages From Common Charts

The table below brings together typical values from several widely used sources. Figures are rounded for readability and may vary slightly between charts and hospitals.

Source Or Chart	Approx Length	Approx Weight
NHS week-18 description	14.2 cm (head to bottom)	190 g
Parent app using crown-rump	14 cm (head to bottom)	200 g
Hospital growth chart (WHO-style)	22.2 cm (head to heel)	222 g
Perinatal reference table	22 cm (head to heel)	223 g
General pregnancy handbook	21–22 cm (head to heel)	210–230 g
Online week-by-week guide	14–15 cm (head to bottom)	190–210 g
Typical clinical “average”	14–15 cm CRL or ~22 cm CHL	around 200 g

If your baby measures a little above or below any of these numbers, the main issue is how growth looks over time. A baby who stays near the same centile across several scans is usually doing well, even if that centile sits on the smaller or larger side.

Why Charts Show Slightly Different Numbers

Even when charts use the same measurement type, they still may not match exactly. That happens because:

• Populations differ: Growth references may be based on babies from one country or region, which can shift averages.
• Study methods differ: Some charts rely on direct measurements, others on equations that estimate weight from several ultrasound markers.
• Updates over time: Older tables may list slightly lower weights than newer research that draws on recent ultrasound data.

For that reason, your midwife or obstetrician will use the growth-chart built into the local system instead of comparing your scan to numbers from random apps. What matters most is whether your baby follows a steady pattern on that chart.

Fetus Size 18 Weeks: What You See On The Screen

An 18-week scan often feels special, because you can see clear body details while baby still fits neatly on the screen. Many people have the detailed “anatomy” or “anomaly” scan between 18 and 20 weeks, where the sonographer checks each area and records size measures for the head, abdomen, and thigh bone.

Guidance from organisations such as the American College of Obstetricians and Gynecologists notes that this mid-pregnancy window is used to confirm growth, look at organ structure, and check dates when needed.

Common Measurements At 18 Weeks

On the scan report, you may see several abbreviations. Each one helps build a picture of size and proportions:

• BPD (biparietal diameter): the width of the head from side to side.
• HC (head circumference): a measure around the outside of the skull.
• AC (abdominal circumference): a line around the tummy, which helps estimate weight.
• FL (femur length): the length of the thigh bone, a useful marker of long-bone growth.

The ultrasound machine runs these values through established formulas to estimate fetal weight. For 18 weeks, that estimate often lands near 200 grams, give or take several dozen grams either way. It is normal for the report to list a range and a centile instead of a single fixed figure.

Sonographers also look at how evenly baby is growing. If the head, tummy, and thigh bone all land on similar centiles, that suggests balanced growth. If one area is much smaller or larger than the others, your team may suggest follow-up scans to see whether that pattern continues.

What The Scan Image Tells You About Size

At this stage, baby’s body looks more in proportion than in the first trimester. The head still makes up a large share of total length, but the limbs have stretched out and hands and feet are easy to spot. On some machines you may see tiny fingers, facial features, and movement such as stretching, swallowing, or thumb-sucking.

Second-trimester references from centres such as Mayo Clinic describe hearing as active around this time, with ears standing out from the head and the digestive system starting regular work. Those changes do not alter length directly, but they show that growth is happening in a coordinated way.

How Fetal Size Changes Between 17 And 20 Weeks

From 17 to 20 weeks, length and weight rise in steady steps. Many charts show an increase of about 1–2 cm in length and roughly 40–60 grams in weight each week during this stretch. The table below gives a simple snapshot based on growth tables that use crown-rump length up to 19 weeks and crown-heel length beyond that point.

Gestational Week	Approx Length	Approx Weight
17 weeks	13 cm CRL or ~20 cm CHL	around 140–160 g
18 weeks	14 cm CRL or ~22 cm CHL	around 190–220 g
19 weeks	15 cm CRL or ~24 cm CHL	around 230–270 g
20 weeks	16 cm CRL or ~25–26 cm CHL	around 280–330 g

These values sit close to ranges reported by national health bodies and specialist fetal growth charts. A baby who tracks along a higher or lower centile than the middle line still often has a healthy outcome, especially when the pattern is consistent and blood flow through the placenta looks good on scan.

What Affects Fetal Size At 18 Weeks?

Two babies at the same stage can differ in size for many harmless reasons. Some families tend to have smaller babies, while others tend to have larger ones. Height, body build, and past birth weights in the family all feed into that picture.

Growth around 18 weeks also depends on how accurate the due date is and how well the placenta and cord are working. Dating based on a first-trimester crown-rump length usually gives a firmer baseline than dates based only on the last menstrual period.

Other influences that can shift measured size include:

• Placental function: Reduced blood flow can slow growth, which is why extra-scans are sometimes arranged.
• Multiple pregnancy: Twins or triplets share space and nutrients, so their growth pattern differs from that of a single baby.
• Parental health conditions: High blood pressure, diabetes, and certain long-term illnesses can change how quickly a baby grows.

Your own team knows your medical history, scan results, and blood tests, so they are best placed to explain how these factors fit together in your case.

When To Talk With Your Doctor Or Midwife

Feeling nervous about baby’s size at 18 weeks is often seen. Most of the time, a measurement that sits a little above or below the textbook average still fits within the normal range when growth from scan to scan is steady.

You should contact your maternity unit, clinic, or doctor urgently if you have heavy bleeding, strong pain that does not settle, leakage of fluid from the vagina, a fever, or any symptoms that feel severe or worrying. Those situations need rapid assessment, no matter what the last growth scan showed.

For other worries, bring your scan report or printout to your next visit or call your midwife sooner if anxiety feels hard to manage. Ask which growth chart the team is using, what centile your baby measures on, and whether they recommend another scan. Hearing how your own measurements line up with that chart gives a clearer picture than comparing single numbers to random websites. Many parents also find it helpful to jot questions in a notes app before the scan so nothing slips their mind during the appointment or afterwards.