Staff at the Bristol hospital where Charlotte Bevan and her four-day-old daughter went missing had no reason to think she would leave.
A spokesman for University Hospitals Bristol said Miss Bevan was due to be released tomorrow "but she elected to leave for reasons unknown to us".
They added: "Charlotte was recovering from the birth of her baby, and there were no indications that she was planning to self-discharge.
"Charlotte and her baby were seen by staff at 8.30pm. When staff went to check on her at 9.05pm they found Charlotte and her baby missing and immediately sounded the alarm.
"We are working closely with other agencies to establish that Charlotte and her baby are safe."
A mum who went missing from a Bristol hospital with her newborn baby had "mental health issues", her sister said.
Janet Tibbs told the Bristol Post: "Charlotte did have mental health issues.
"The police spoke to me this morning, I couldn't think of anything to say, I was just rambling on about what we did the last time I saw her. I don't think it was much use."
She added Charlotte and her boyfriend Pascal Malbrouck were "really looking forward" to the birth of their first child.
The 30-year-old walked out of a maternity ward with four-day-old Zarnee Teanna last night.
Her boyfriend confirmed Charlotte suffered from depression and schizophrenia but said she had been happy before the birth.
He said he could think of no reason why she would go missing.
"We all love and care about you Charlotte. Please just let someone know you are safe. We want you to come home."
Police searching for missing Charlotte Bevan and her four-day-old baby daughter are looking in bins and undergrowth around St Michael's Hospital in Bristol.
Police are investigating a number of sightings of missing mum Charlotte Bevan.
A spokeswoman for Avon and Somerset Police said: "We are investigating a number of sightings of Charlotte and one of these sightings was in Royal Fort Road. We are currently looking into that sighting."
They would not say what other leads they are following up on but searches are continuing around the hospital grounds, including in undergrowth.
Police have extended the search for a missing mother and new born baby to undergrowth around St Michael's Hospital in Bristol.
The devastated partner of missing mum Charlotte Bevan has made an emotional appeal for her safe return.
Charlotte was last seen at around 9pm yesterday at St Michael's Hospital in Bristol with her four-day-old daughter.
Her partner Pascal Malbrouck said: "I left the hospital at 8.30pm and they say she left between 8.45 and 8.55pm.
"Nobody saw anything, nobody say nothing. They said she went out of this door and I counted seven cameras but nothing. They weren't working. She couldn't have walked far, she went out in flip flops with a baby in her arms.
"If somebody had seen something they should have reported it by now, I just don't know."
He also wrote on his Facebook page: "My daughter has gone missing with her mum Charlotte Bevan, Please if anyone has got any info or has seen her please contact me."
A mum who went missing from a Bristol hospital with her newborn daughter looked at positive thinking videos hours before she disappeared.
Charlotte Bevan's Facebook page said she watched a video called Get Through Positive and Negative Thinking Rough Patches at 7.16am yesterday.
She left St Michael's Hospital around 13 hours later later with four-day-old Zarnee Teanna.
Her Facebook also recorded she listened to the R Kelly Song Ignition at around 2pm yesterday.
On the page she describes herself as a "free spirit". It adds: "I am a free open minded individual. I love music, art, dancing and making clothes...I appriciate (sic) people for who they are kind or nasty they all need love."
Avon and Somerset Police have cordoned off Royal Fort Road near St Michael's Hospital.
30 year old Charlotte Bevan was last seen leaving the hospital at 9pm yesterday. Ms Bevan and her 4 day old baby have not been seen since.
The police have closed off the road after one of a number of possible sightings.
Police are appealing for friends of missing mum Charlotte Bevan to come forward to let them know her and her four-day old baby Zarnee Teanna are okay.
Ms Bevan has not been seen for 12 hours after disappearing from Bristol Maternity Hospital at around 8.30pm last night.
Avon and Somerset Police asked anyone Charlotte may have contacted to get in touch with them immediately by calling 101.
We're appealing to friends of Charlotte who may have been contacted by her to let us know she and baby are okay. Call us on 101.
The disappearance of a mother and her newborn baby is "out of character", friends said.
Charlotte Bevan left Bristol Maternity Hospital with her four-day-old daughter Zarnee Teanna between 8.30pm and 9pm last night.
Police are urgently appealing for her to come forward or for information on her whereabouts.
Writing on Avon and Somerset Police's Facebook page, Isaac Jack McCardle, said: "I know her and her partner. This is out of character.
"I really hope they are both safe and are all reunited soon. I do feel out of respect people should stop jumping to conclusions. They are a happy couple."
He added he had helped search for Ms Bevan and was "hoping she may have got some transport somewhere to a safe place."