(CNN) Zulmarys Molina arrives at the high-risk pregnancy clinic in San Juan, Puerto Rico, shortly after dawn, excited to see her baby.
Inside the examining room, she raises her shirt so Dr. Alberto de la Vega can put an ultrasound wand on her belly.
He heads straight for the baby's brain.
When Molina was three months pregnant, she got the Zika virus from a mosquito. It wasn't a big deal for her -- she just felt sick for a few days.
But it could be devastating for her baby. Zika can have an affinity for a fetus's brain tissue, eating away and destroying it.
Now five months pregnant, Molina anxiously watches the ultrasound monitor as de la Vega performs his examination on her daughter, whom she's named Micaela.
"This is a very stressful situation," Molina says. "You don't want to think about it, but you're always reminded of what Zika can do to your baby."
As Zika continues to spread, this scene is being repeated over and over again at doctors' offices throughout many parts of the Caribbean and Latin America.
More than 950 cases of fetal malformations potentially associated with Zika have been reported worldwide, according to the World Health Organization.
And experts say it's only a matter of time before it happens in the continental United States, too.
De la Vega moves the wand carefully, taking several measurements of Micaela's brain. He carefully calibrates the thickness of her cortex, the brain's outer layer of neurons, and the size of the corpus callosum, the band of nerves that connects the brain's two hemispheres.
"Her brain anatomy looks normal," de la Vega says.
You can feel the tension in the room subside as a huge smile spreads across Molina's face.
De la Vega takes a break from measuring the brain and shows Molina 3-D images of her daughter's face and beating heart. They both look perfect.
"She has my nose!" Molina notices, still smiling.
Then he goes back to the brain to take one more measurement. And something in his face tells you it's not quite right.
He measures the circumference of Micaela's head.
"Your baby's head growth is lagging behind," he tells her. "Given the situation, there's reason to be concerned."
The "situation" is that Zika has been linked to microcephaly, a condition where babies have unusually small heads and underdeveloped brains.
When Molina was 18 weeks pregnant, Micaela's head size was almost average -- somewhere between the 40th and 50th percentile compared with other babies, de la Vega says.
But today, at 22 weeks, it's below the 20th percentile.
This might just be a fluke, he explains to Molina. Babies' heads grow at unsteady rates, and she might speed up soon. Plus, because of Micaela's position, it was hard to measure her head. He repeats that it's reassuring that the structures inside Micaela's brain look normal.
He asks Molina to come back in two weeks for another ultrasound. That will help him know if today's number was a one-off or a sign of brain damage.
It doesn't matter to Molina what they find in two weeks. She's already decided she'll keep her baby, no matter what.
She knows that it will be more difficult to care for Micaela if she has severe brain defects, but Molina knows about hard work. Her income as a university admissions officer supports herself, her 2-year-old son and her mother, who live together in an apartment in public housing.
Raised Catholic and now a practicing Pentecostalist, Molina says her religious beliefs will not allow her, under any circumstances, to terminate the pregnancy.
Other women in her situation have made different choices.
On March 30, the day before Molina had her most recent ultrasound, the New England Journal of Medicine published a report about a woman with Zika in Washington who chose to terminate her pregnancy because the fetus had severe brain defects.
So far, there have been 32 pregnant women in the United States who've had Zika, according to the Centers for Disease Control and Prevention.
These women were not bitten by mosquitoes at home. They contracted Zika by traveling to an area where the virus is spreading or by having sex with a man who had traveled, according to Tom Skinner, a CDC spokesman.
Zika virus 'scarier than initially thought'
But in the coming months, as the weather gets warmer, experts anticipate that mosquitoes will start biting pregnant women in the continental United States, too.
"It's not a matter of whether Zika will come here, but when," said Dr. Jose Cordero, a professor of public health at the University of Georgia and the former director of the CDC's National Center on Birth Defects and Developmental Disabilities.
This means mothers here will have to make decisions like the one Molina has made.
Abortion decisions are never easy, but with Zika, there's an added element of difficulty: The brain damage caused by the virus doesn't seem to show up until relatively late in the pregnancy, when it's more complicated, medically and emotionally, to end a pregnancy.
With the patient in Washington, her baby's brain defects didn't appear until 19 weeks, nearly halfway through the pregnancy. The parents decided to terminate two weeks later.
"This was a very difficult decision for them to make," said the woman's obstetrician, Dr. Rita Driggers, an associate professor of gynecology and obstetrics at Hopkins.
With another patient, also reported in the New England Journal of Medicine, the defects didn't show up until even later -- 29 weeks. That mother also chose to terminate her pregnancy.
Molina's next ultrasound is April 13. She awaits it with some anxiety.
"But I am feeling very positive," she says. "I know everything will be OK."
CNN's Debra Goldschmidt and John Bonifield contributed to this report.