Nathan Fox

2 min Read Nathan Fox

Nathan Fox

How Worried Should I Be About Microcephaly If My Baby’s Head Is Measuring Small?

Q&A on ultrasounds

Nathan Fox

2 min Read

My two recent ultrasound head circumference measurements have shown my baby’s head to be in or under 1%, which technically is microcephaly. HC was 7% at 28 weeks, then 1% at 30 weeks, then <1% at 32 weeks. The rest of the baby’s measurements are normal: femur is 65% and torso is 46%, estimated weight is on track. My OBG doesn’t seem too concerned and only says the measurement could be off. How accurate are HC measurements by ultrasound, and how concerned should we be about microcephaly or major developmental issues? It feels like we are begging for follow-up scans or MRI to get an accurate reading, while my OB is just assuming the two scans in a row are wrong.

—Panicking at 32 weeks

Microcephaly is a severe condition of abnormal fetal brain development, and it is associated with several long-term consequences. However, most fetuses with small head sizes do not have microcephaly, even those with a head circumference less than the first percentile. Unfortunately, this is one of the quirks with many of the software programs used to generate percentiles from HC measurements. 

Microcephaly is suspected when the fetal HC is more than two standard deviations (SD) smaller than the expected. Many, if not most, fetuses with an HC less than the first percentile do not meet this criteria. Meaning the percentile often seems scarier than necessary. 

I would recommend getting an ultrasound evaluation by a maternal fetal medicine specialist and asking them specifically (a) if the HC measurement is more than two SD smaller than the mean, (b) if there are any other concerns on the ultrasound, and (c) if there is any suggestion of fetal microcephaly. When there is that suspicion, fetal MRI and/or amniocentesis are often recommended.

One online tool for examining the risk of fetal microcephaly from the HC measurement can be found here.


What I take from Nate’s words is a combination of reassurance and action (which is usually what I expect from him). There is a very high likelihood — even with the measurements you’ve seen — that everything is fine. But it likely does make sense to follow up with an ultrasound that is focused on this issue and that gives you a measurement that is more specific than a percentile.

Very best of luck, and please keep me updated. —Emily

Community Guidelines
0 Comments
Inline Feedbacks
View all comments
Stacks of diapers, cloths, rubber ducks, and other baby supplies.

Apr. 21, 2023

2 min Read

How Can I Prepare for My Baby’s Cleft Palate?

My baby was recently diagnosed with a cleft lip and palate at the 20-week ultrasound. Any perspective on how to Read more

An ultrasound view of the bottoms of a baby's feet.

Jul. 19, 2023

2 min Read

How to Understand My Baby’s Clubfoot Diagnosis

Do you or your network have advice for parents dealing with clubfoot? At our 20-week ultrasound, we found out our Read more

a doctor conducting and ultrasound in the clinic

Aug. 23, 2023

4 min Read

Are Ultrasounds Safe?

Have you done any research into the safety of ultrasounds (both scans and their use to monitor how things are Read more

A doctor measures the fundal height on a pregnant person.

Nov. 6, 2023

9 min Read

How Big Will My Baby Be?

It’s been a few years, but when I think back on my doctor’s visits during pregnancy, the most vivid memories Read more