Singapore's Health Minister Gan Kim Yong said on Wednesday night that the pregnant woman was living in the same housing block where one of the neighbours had tested positive for the virus.
The woman has been admitted to hospital where doctors are monitoring her and her fetus' condition, Yong added.
The minister also confirmed 24 new cases of Zika, including two that were registered outside the districts where the infection had previously been identified.
"Over time, we expect Zika cases to emerge from more areas. We must work and plan on the basis that there is Zika transmission in other parts of Singapore and extend our vector control efforts beyond the current affected areas," Yong said.
Several countries, including the US, South Korea and Taiwan, issued travel alerts to Singapore after the outbreak was first detected on Saturday.
Zika is transmitted by the mosquito Aedes aegypti and causes symptoms similar to dengue.
The virus can cause chronic arthritis, and has been linked to fetal malformations.
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