The images pouring out of Brazil are haunting: struggling newborns with misshapen heads, cradled by mothers who desperately want to know whether their babies will ever walk or talk.
There are thousands of these children in Brazil, and scientists fear thousands more might come as the Zika virus leaps across Latin America and the Caribbean. But the striking deformity at the centre of the epidemic, microcephaly, is not new: it has pained families across the globe and mystified experts for decades.
For parents, having a child with microcephaly can mean a life of uncertainty. The diagnosis usually comes halfway through pregnancy, if at all. The cause may never be determined - Zika virus is only suspected in the Brazilian cases, while many other factors are well documented. And no one can say what the future might hold for a particular child with microcephaly.
For doctors, the diagnosis means an ailment with no treatment, no cure and no clear prognosis. If the condition surges, it will significantly burden a generation of new parents for decades.
Dr Hannah Tully, a neurologist at Seattle Children's Hospital, sees the pain regularly, particularly among expectant parents who have just been told that an ultrasound showed their child to be microcephalic: "a terrible situation with which to be confronted in a pregnancy," she said.
An estimated 25,000 babies receive a microcephaly diagnosis each year in the United States. Microcephaly simply means that the baby's head is abnormally small - sometimes just because the parents themselves have unusually small heads.
"By itself, it doesn't necessarily mean you have a neurological problem," said Dr Marc Patterson, a paediatric neurologist at the Mayo Clinic Children's Center.
But microcephaly can portend significant brain damage, as well. The most severe cases can be detected before birth with ultrasound scans, but usually only toward the end of the second trimester, at about 24 weeks.
Most expectant mothers have ultrasound exams at about 20 weeks, however, so the condition can be missed. Many parents learn their child is microcephalic only after birth, when the newborn's head is measured.
Even when it's made early, the diagnosis raises hard questions. Abortion is generally legal in the United States only until the fetus is viable outside the womb, which can range from 24 to 26 weeks.
That leaves parents little time for an enormously difficult decision, complicated by doctors' inability to say what the effects of microcephaly might be.
Prognoses for these children vary widely. At least 10 per cent have no mental deficits at all; others are highly functional, albeit with intellectual disabilities.
Still others are profoundly disabled, in wheelchairs with limited ability to communicate, and fed through a gastric tube.
"Families have very little time to have the necessary studies, get the results, process their thoughts and make a decision before they reach the legal limits of termination," Dr Tully said.
Melissa and Peter Therrien, of Massachusetts, faced that choice when they learned that their daughter had a very small skull, after an ultrasound during the 24th week of pregnancy.
"I felt heartbroken," Melissa said. "The doctor gave me the option to terminate the pregnancy." But, she said, "I couldn't do that."
Their daughter, Alainah, is now 15 months old, but her development is uncertain. She can walk, although doctors said she might not, and she is given to peals of laughter.
Yet Alainah speaks just three words: Mama, Papa and "aba," which she uses to describe various objects. She can use sign language to say she is hungry. She has passed some standard milestones, but her parents do not know how far she will progress. Doctors at Boston Children's Hospital will not know the extent of damage to her brain for nine months or so. She may still have seizures and profound disability.
"It's just tough," said dad Peter. "There's nothing we can do to fix it. We'll pretty much be walking on eggshells for the rest of our lives."
If the cause of microcephaly is determined, it can give some clues to how children will fare.
Certain genetic conditions are linked to microcephaly, among them Down syndrome. Pregnant women who are badly malnourished, have diabetes or consume alcohol are also more likely to have children with microcephaly.
Microcephaly can develop after birth in children with some genetic disorders, and in some infants deprived of oxygen during labor whose injured brains stop growing in their first few years.
But of particular concern to researchers now is that microcephaly can be caused by infections, including toxoplasmosis, German measles and cytomegalovirus, a ubiquitous virus.
Zika virus may soon join the list. If it does cause microcephaly, it is a rare complication. No increases in microcephaly are conclusively linked to the virus outside Brazil, although French Polynesia is investigating a small number of neurological problems in babies after an outbreak there.
The Therriens alternate between optimism and pessimism, waiting for the tests that will provide clues to Alainah's course.
"My biggest worry is her future," said Melissa, who also has a 6-year-old daughter without microcephaly.
"Is she ever going to be able to live a life like her sister will?"