Health authorities are investigating whether three babies born with smaller than usual heads in non-Zika watch zones have been affected by the mosquito-borne virus.
They are also checking on a 36-week unborn baby in a Zika watch area that also has an unusually small head.
Doctors noticed in an ultrasound scan on the unborn baby that its head was smaller than usual, which raised concern over whether it had microcephaly, a birth defect associated with incomplete brain development that is found to be associated with the Zika virus.
Dr Amnuay Gajeena, director-general of the Department of Disease Control, said yesterday the baby is carried by one of 33 pregnant women in Zika watch areas that are being monitored under the Zika virus infection surveillance programme.
Dr Amnuay said the three new babies were found to have an abnormality only after they were born.
A team of obstetricians, gynaecologists and paediatricians was conducting further tests to determine whether their unusually small heads was microcephaly associated with Zika virus infection, he said.
This week, he said, the results of the four suspected microcephaly cases will be considered by an academic committee led by department specialist Emeritus Prof Dr Prasert Thongcharoen.
Dr Amnuey said a baby born with a smaller than usual head could have been affected by something other than the Zika virus.
He allayed public concerns over the four cases, saying the Zika virus had not become endemic in Thailand. The risk of a Zika virus infection resulting in microcephaly ranges from 1% to 30%.
"In this regard, the Public Health Ministry is monitoring about 30 pregnant women, and the chance of detecting microcephaly here can be three to four cases, which is usual," he said.
The US had discovered about 10 such cases while Brazil has reported thousands of cases already, he said.
According to the Bangkok Metropolitan Administration's health office, the Thai capital between Sept 1 and 21 had recorded 26 Zika infection cases involving 10 males and 16 females.
Sathon district reported the highest number of Zika cases with 13, followed by Kannayao and Thon Buri districts with three and two cases. Suan Luang, Bang Phlat, Din Daeng, Ratchathewi, Chom Thong, Bang Rak, Bang Khunthian, Phaya Thai each recorded one Zika case.