Abnormalities have shown up anywhere from two to twenty-nine weeks after symptoms

During pregnancy, providers can monitor a fetus for signs of **congenital Zika
virus infection**. The practical question is when to look, and the honest
answer is that it is not known.

Serial ultrasound

For pregnant women with confirmed or possible Zika virus infection, **serial
fetal ultrasounds every 3–4 weeks should be considered** — to assess fetal
anatomy, particularly neuroanatomy, and to monitor growth. Scans should
include **detailed fetal anatomy, to detect brain or associated structural
abnormalities that might occur before microcephaly.**

That last clause matters: microcephaly is the recognised consequence, and it
is a late sign. The point of detailed anatomy is to see what precedes it.

The uncertainty, stated plainly

**Insufficient data are available to define the optimal timing between Zika
virus exposure and initial sonographic screening. Abnormalities have been
detected anywhere from 2 to 29 weeks after symptom onset.**

A twenty-seven-week window. Brain abnormalities have been identified by
ultrasound in both the second and third trimesters in case reports.

With no data to fix the timing, the guidance says providers **may consider
extending the interval between ultrasounds in accordance with patient
preferences and clinical judgement** — an unusually candid handover of a
decision that evidence cannot yet make.

How accurate is it

**The accuracy of ultrasound for detecting brain abnormalities in Zika
infection is not known**, and depends on:

• Timing of infection relative to screening
• Severity of the abnormality
• Patient factors, such as obesity
• Gestational age

Which means a normal scan is reassuring rather than conclusive, and a family
should be told that before it is done rather than after.

Source: Centers for Disease Control and Prevention.