New doubts have been cast on whether the Zika virus is indeed a cause of microcephaly in newborns.

The preliminary results of a large study of pregnant Colombian women infected with Zika showed that between March 28 and May 2 this year, of the nearly 12,000 pregnant women with clinical symptoms of Zika, no cases of microcephaly were reported.

According to the preliminary results of the study that were published by the New England Complex Systems Institute (NECSI), the numbers of microcephaly cases reported in Columbia do not add up.

The NECSI said: "In Brazil, the microcephaly rate soared with more than 1,500 confirmed cases."

However, the NECSI said that of the 50 microcephaly cases reported in Colombia, only four have been connected with Zika.

"If Zika is to blame for microcephaly, where are the missing cases? Perhaps there is another reason for the epidemic in Brazil," the NECSI said.

Considering the evidence presented in this study, the NECSI says that the cause of microcephaly in Brazil should be reconsidered.

The Zika outbreak first emerged in Brazil in mid-2015. The mosquito-borne virus was subsequently linked to birth defects in children.

Microcephaly, a congenital condition associated with incomplete brain development, is one of these defects.

Infants with microcephaly are delivered with heads that are smaller than expected, in comparison to other babies the same age.

While Zika infections are, at present, widely considered to be the primary cause of microcephaly, the NECSI suggests that the use of the pesticide pyriproxyfen could be the cause of microcephaly cases.

Pyriproxyfen is applied to drinking water in some parts of Brazil to kill the larvae of the mosquitoes that transmit the Zika virus.

The NECSI and the Swedish Toxicology Sciences Research Centre, a physicians' group in Brazil and Argentina, have now called for more studies to be done regarding the potential link between the use of pyriproxyfen and microcephaly.