Data on more than 16,000 babies born in northeastern Brazil over the last four years suggests that the relationship between the Zika virus and the spike in births of babies with shrunken heads is more complex than previously thought. So far, no one has presented a clear explanation of how the two relate. However, a report to the World Health Organization suggests we are a long way from understanding what is happening.

In late 2015, a dramatic rise was reported in babies born with microcephaly, or an abnormally small head, in northeastern Brazil. The coincidence, both in time and location, with an outbreak of the Zika virus was so strong that it has generally been accepted that infection with the virus during pregnancy is the cause. Nevertheless, no definitive link has been found.

However, cardiologist Dr. Sandra da Silva Mattos has now produced evidence that the problem started earlier than reported. Mattos is the leader of a team researching heart defects in babies born in Paraiba, a Brazilian state that recorded the second highest number of microcephaly cases last year. As part of her work, she had nurses collect data on 16,208 Paraiban babies born since 2012, including head circumference.

Given the urgency of the problem, Mattos rushed to submit her results to the WHO bulletin, where it has been published without waiting for peer review. This is common practice during public health emergencies, but increases the possibility of an undetected error.

Nevertheless, what Mattos reports is astonishing, and currently inexplicable. Her data shows that at least as far back as 2012,  microcephaly has been almost a thousand times more common than normal for babies born in Paraiba during the spring and summer. Depending on the criteria used to define microcephaly, between 2 and 8 percent of babies born since 2012 qualify, with a peak of 12 percent in mid-2014. If Zika is the cause, this contradicts the theory that the virus reached Brazil in 2014.

Even if the virus reached Brazil years before detection, it would not explain Mattos' observation of changing severity. Before 2015, most babies born with the condition had heads only slightly smaller than normal – a difference detectable through the measurements of head circumference Mattos had nurses collect, but not noticeable otherwise.

In 2015, however, many of the affected children had skulls so small that the condition was immediately observable, which is how the problem came to light.

In an interview with ABC radio, Mattos said, “You see in the most severe cases of microcephaly, that group which are the very severe cases, they are definitely increasing from the last part of 2015.”

Moreover, while microcephaly is usually accompanied by defects in other organs, Mattos said that most of the cases that are currently being reported are very severe microcephaly without anything else.

Public health officials were already struggling to understand why Zika, known for 50 years as a relatively harmless virus, should suddenly start to cause microcephaly. Mattos' work suggests it may be exacerbating a pre-existing condition, or the cause is something else entirely.