A 25-year-old Florida woman who had been missing for months was found on Wednesday under dramatic circumstances, after she gave birth on a subway train rumbling beneath the streets of Manhattan.

The woman, Jenny Saint Pierre of Hallandale Beach, Fla., had been reported missing by her family, who said they had not seen her since last summer. On Wednesday, a law enforcement official identified her as the woman who gave birth on a southbound W train in Midtown shortly before noon.

A police spokesman said the mother and the baby, a girl, had been brought to Bellevue Hospital in stable condition. Ms. Saint Pierre’s sister, Stephania Saint Pierre, confirmed the mother’s identity after seeing a video of the newborn’s first moments that was shared on social media on Wednesday.

“Oh, my God, look at her little face!” Stephania said in a phone interview from her home in Texas as she watched the video. She recognized her sister’s pink duffle bag on a subway seat and heard her voice as another passenger lifted the infant. “Oh, my God, I am going to cry! That’s my first niece!”

Stephania was surprised to learn that her sister was in New York City and said she did not know why she had gone there.