She was three months pregnant and had a 10-month-old toddler running a high fever when the authorities caught her taking the drug Ice.
The Singaporean woman was one of 117 suspected drug offenders arrested by the authorities in an 11-day operation that ended on Friday morning.
The Central Narcotics Bureau (CNB) said that drugs with a street value of about $52,000 were seized during the operation which began on Jan 21.
They included Ice, heroin, cannabis, Erimin-5 tablets, Ecstasy tablets, ketamine and new psychoactive substances.
The CNB operation, which was assisted by the police, covered areas including Bedok, Buangkok, Bukit Batok, Commonwealth, Jurong, Pasir Ris, Serangoon, Tampines and Yishun.
The 31-year-old mother was arrested on Thursday with her 36-year-old partner near Balestier Road, the CNB said.
They had been moving frequently from one hotel to another to evade arrest.
The toddler was taken to the hospital.
The drug Ice and various drug paraphernalia were recovered from the room rented by the couple, the CNB added.
In another case, a 31-year-old Singaporean man was arrested near Toa Payoh Lorong 1 on Jan 23.
Officers recovered about 84g of Ice, many empty plastic sachets, a digital weighing scale and drug-taking equipment from the residential unit where he was found.
Investigations are continuing.