Some women should get blood tests, and just about all should get ultrasound scans.
On Jan. 19, the C.D.C. issued interim guidelines for women in that situation and for their doctors. The guidelines are complex — and may change.
In general, they say that pregnant women who have visited any area with Zika transmission should consult a doctor. Those who have had symptoms of infection like fever, rash, joint pain and bloodshot eyes during their trip or within two weeks of returning should have a blood test for the virus.
That recommendation is controversial, because even women with no symptoms may have been infected — 80 percent of those who get the virus do not feel ill — and there is no evidence that babies are hurt only when the mother has been visibly ill. But at the time the guidelines were issued, the C.D.C. and state health departments simply did not have the laboratory capacity to test every pregnant woman who visited Latin America and the Caribbean in the last nine months, as well as every pregnant woman in Puerto Rico.
Even for women who get blood tests, the news is not entirely reassuring. Tests for the virus itself only work in the first week or so after infection. Tests for antibodies can be done later, but they may yield false positives if the woman has had dengue, yellow fever or even a yellow fever vaccine.
Under the C.D.C’s testing algorithm, pregnant women who have been to affected regions – whether they have symptoms or not, and whether they have negative or positive blood tests – should eventually have an ultrasound scan to see if their fetuses are developing microcephaly or calcification of the skull.
Unfortunately, an ultrasound usually cannot detect microcephaly before the end of the second trimester.
Some women also should have amniocentesis to test the fluid around the fetus for Zika virus. But amniocentesis involves piercing the amniotic sac with a long needle through the abdomen; it is slightly risky for the fetus and is not recommended before 15 weeks gestation.
Several companies are working on rapid tests for Zika infection. The C.D.C. also usually distributes test kits and training materials to state health departments during outbreaks, which should increase testing capacity.