Police said they had registered a case of murder and gang rape after the alleged attack on May 29 in Gurgaon, on the outskirts of New Delhi.
"We are interrogating several people and should be able to progress further in the case today, including making possible arrests," Gurgaon's commissioner of police Sandeep Khirwar told AFP.
He said the baby had died of head injuries, and estimated the mother was aged 19 or 20.
The woman told police she had been attacked after getting into an autorickshaw with her daughter around midnight on May 29 to travel to her parents' house in Gurgaon, in northern Haryana state.
There were already two men on board, along with the driver.
India has a gruesome record on rape, with the capital New Delhi alone registering 2,199 rape cases in 2015 -- an average of six a day.
Nearly 40,000 rape cases are reported every year but the real number is thought to be much higher, with victims wary of how their complaints will be dealt with or the social stigma attached to sex crimes.
India strengthened its laws on sexual violence after the fatal gang rape of a Delhi student in 2012 caused global outrage, but attacks on women are still widespread.
The latest incident comes shortly after two other high-profile rape cases in Haryana.
Last month, police arrested a man accused of kidnapping, raping and killing his ex-girlfriend in the state. He is alleged to have smashed her head with bricks before running her over in a car.
In another case, a 10-year-old was found to be five months pregnant after allegedly enduring sustained sexual assault at the hands of her stepfather.
A court later granted the girl permission to have an abortion despite Indian law prohibiting terminations beyond 20 weeks.
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