Where is Kierra Coles? The long-missing, pregnant Chicago postal worker was due to give birth last week

Last fall, Kierra Coles was in a really good place.

Three months pregnant with her first child, Coles "was happy," her mother, Karen Phillips, told ABC News. "She was excited about being a mother; we were all excited." Mother and daughter talked every day.

Then, on Oct. 2, 2018, the 26-year-old stepped out of her home on Chicago's South Side in her postal worker uniform and disappeared into thin air.

"I have no idea what happened," Phillips told ABC News on Saturday.

Chicago Police Department officials said that "foul play is suspected."

Coles is the fourth born of Phillips' five children and they talked everyday.

During their last phone call on Oct. 1, 2018, Coles asked for her mother's advice on a product and "she seemed okay."

Coles due date was April 16 and while the sex of the unborn baby was unknown, Phillips said her daughter was sure she was having a boy.

The following day, police said, Coles was seen on surveillance video leaving her apartment near 81st Street and Vernon Avenue. Her vehicle was found in front of her apartment with her purse, cell phone, and a packed lunch still inside, ABC Chicago station WLS reported.

"We talked everyday so I tried calling her in the morning, she didn't answer," said Phillips, 47, who assumed Coles was already at work.

"I called in the afternoon, again, no answer," said Phillips, who guessed Coles probably wasn't feeling well. "Then I called in the night, no answer and the next day, no answer."

By Oct. 4, 2018, not hearing from Coles for more than 24 hours concerned Phillips. The grandmother-to-be said she went by Coles' apartment where she saw her daughter's car still parked.

"I called her job and they said she didn't report to work so I called the police and asked them to do a wellness check," Phillips told ABC News.

Phillips said she also called Coles' boyfriend of six years, who is also the expected father-to-be.

"Kierra was going to name the baby a junior -- she always said that. If it's a boy she'll name him a junior," said Phillips.

Anthony Guglielmi, a spokesman for the Chicago Police Department, told ABC News via email that the investigation into Coles' disappearance continues, in coordination with the Federal Bureau of Investigation (FBI) and the U.S. Postal Inspection Agency.

"We have interviewed several people of interest who may have been among Individuals to last see Ms. Coles," Guglielmi wrote. "Since her disappearance, there have not been any confirmed sightings or communication from her. We continue to actively follow up on leads and are seeking any information in relation to her disappearance."

A U.S. Postal Inspection Service spokesman said the agency is offering a $25,000 reward posted for any information learning to finding Coles.

It's now been more than 10 days since Coles was expected to give birth, and Phillips said she tries to remain positive as her suspicions run high towards her daughter's boyfriend.

"I never pointed fingers at him in the beginning, but all this time as passed...I haven't heard anything from him since the day the police went to do the wellness check. I haven't seen him around town, nothing," said Phillips, adding, "He must have something to do with it and this."

Guglielmi confirmed with ABC News that Coles' boyfriend remains a person of interest "given he may have been one of the last individuals to see her," but wouldn't comment any further.

Numerous efforts by ABC News to reach the boyfriend were unsuccessful.

Coles is described as having black hair and brown eyes. She stands at 5 feet 4 inches tall and weighs 125 pounds.

Anyone with information is urged to call the Chicago Police Department at 312-747-8274.