What If Baby Measures Smaller Than Expected?

Being told your baby is measuring smaller than expected can be worrying, but there are many reasons for this — and most of them are completely benign. Here's what it means and what happens next.

Table of Contents

1. What "Measuring Smaller" Means
2. Common Reasons for a Small Measurement
3. What Happens After the Scan
4. When It's a Concern
5. What You Can Do
6. Will You Need More Scans?

What "Measuring Smaller" Means

At the dating scan (12 weeks), a difference of up to 5-7 days from your expected dates is considered normal. If the difference is more than 7 days, your due date may be adjusted. At earlier scans, a difference of up to 5 days is usually within normal limits.

It's important to know that "measuring small" doesn't mean your baby is unhealthy — it usually means your dates are slightly off or your baby is simply on the smaller side of normal.

Common Reasons for a Small Measurement

• Incorrect dates: This is the most common reason. If your cycle was longer than 28 days, you ovulated later than day 14, or you misremembered your last period, you may be a few days less far along than you think.
• Late ovulation: Even with a regular cycle, ovulation can be delayed by stress, illness, travel, or just natural variation.
• Small but healthy baby: Some babies are just naturally smaller, just as some adults are shorter. If both parents are petite, the baby may be too.
• IVF dating differences: In IVF pregnancies, the dates are very precise, but even so, embryos grow at slightly different rates.
• Earlier than expected at early scans: At 6-8 weeks, dating is less precise and small differences are common.
• Poor growth (IUGR): In some cases, the baby isn't growing as it should due to placental problems, maternal health conditions, or other factors. This is more of a concern in later pregnancy than at the 12-week scan.
• Incorrect measurement: While rare, the measurement can sometimes be slightly off if the baby is curled up or in an awkward position.

What Happens After the Scan

1. Re-measure carefully: They'll take several measurements to ensure accuracy. If the baby is in a difficult position, they may wait for the baby to move.
2. Compare to your dates: If the difference is less than 7 days (at 12 weeks) or 5 days (at 6-8 weeks), it's usually within normal limits. Your due date may be adjusted.
3. Check the heartbeat and other structures: If everything else looks healthy, a small measurement is less concerning.
4. Arrange a repeat scan if needed: If the difference is significant or there are other concerns, you may be asked to come back in 1-2 weeks to check growth.
5. Adjust your due date: If the CRL clearly indicates a different gestational age, your due date will be changed. This is common and nothing to worry about.

When It's a Concern

• The baby measures more than 7 days behind at 12 weeks: This may indicate a growth issue, and a repeat scan will be arranged.
• The baby doesn't grow between scans: If a repeat scan shows no growth, this is a red flag. Your healthcare team will investigate further.
• Other abnormalities are present: If the baby has other structural concerns alongside the small size, a specialist referral may be needed.
• You have risk factors: If you have high blood pressure, diabetes, or a history of growth-restricted babies, your team will monitor growth more carefully.

In the first trimester, the most common reason for a small measurement is simply incorrect dating. True growth restriction is rarely diagnosed this early.

What You Can Do

• Attend all your scans: Follow-up scans are crucial to check growth over time. Don't skip them.
• Eat well: A balanced diet with plenty of nutrients supports your baby's growth. See our nutrition guides.
• Take your supplements: Folic acid, vitamin D, and any prescribed supplements are important.
• Don't smoke: Smoking is a major cause of poor fetal growth. If you smoke, ask your GP for help quitting.
• Rest: While bed rest isn't usually recommended, reducing stress and getting enough sleep is sensible.
• Ask questions: If you're worried, ask your midwife. Don't suffer in silence.

Will You Need More Scans?

If the measurement is significantly small or there are other concerns, you may be offered:

• A repeat growth scan in 1-2 weeks to check the baby is growing
• Additional monitoring throughout pregnancy
• A referral to a fetal medicine specialist if there are structural concerns

Most women whose baby measures slightly small go on to have healthy pregnancies and babies. The key is monitoring to ensure growth continues.

Frequently Asked Questions

Yes. If the CRL at your dating scan differs from your dates by more than 5-7 days, your due date will be adjusted based on the scan measurements. This is standard practice.

Does a small baby mean I'll have a premature birth?

No. A baby that measures small in the first trimester is usually just small for its gestational age or the dates are off. Premature birth is more related to later pregnancy complications, not early scan measurements.

Should I eat more if my baby is measuring small?

Not necessarily. Eating more won't directly make your baby bigger in the first trimester. A balanced, nutrient-rich diet is more important than extra calories. Talk to your midwife about your specific situation.

Further reading:

• NHS — Antenatal care and screening

This article is for informational purposes only and does not constitute medical advice. Always consult your healthcare provider.

Last reviewed: June 2026

Related articles:

• Baby Measures Bigger Than Expected
• CRL Explained
• Reading Your Ultrasound Report
• 12-Week Scan Guide