A search is underway for an 18-year-old woman who disappeared a week ago from Rehoboth, Massachusetts, police and her family said.

       Kylee Monteiro is 11 weeks pregnant and has been missing since Aug. 7, her sister told ABC News.

       "This is not like her at all. This is completely unusual and very scary, to say the least," Faith Monteiro said Thursday.

       Kylee Monteiro, who had just graduated from Attleboro High School two and a half months ago, was last seen in the area of County Street in Rehoboth, Massachusetts, the Rehoboth Police Department said in a press release on Monday.

       Her sister said the 18-year-old had been living at a homeless shelter for pregnant women in Pembroke, Massachusetts, for the past few weeks as her and her boyfriend were "on and off" and that there had been "domestic issues" in the relationship, including an altercation that allegedly occurred the night before she disappeared.

       After an alleged argument on Aug. 6, Kylee Monteiro allegedly called her sister, along with several others, saying the dispute had calmed down but she was fearful for her life, her sister said.

       "She was desperate for help. She was calling out to friends, family, anyone she could, telling them that she was in danger. The whole situation is very scary," Faith Monteiro told ABC News.

       Faith Monteiro said she told the 18-year-old that she could pick her up, but she declined.

       Her sister said the teen is "not the type of person" to ask a stranger for a ride and "doesn't know how to survive on her own."

       The Rehoboth Police Department and the Bristol County District Attorney's Office did not immediately respond to ABC News' request for comment.

       Her sister said Kylee Monteiro was "really excited" to become a mom.

       "She even had a name picked out for the baby. If she had a girl, she was going to name the baby Eleanor," her sister said.

       The teen's whole family is "worried sick" and "just want her to come home immediately," Faith Monteiro said.

       "I love her, I miss her," she said.

       The teen is described as approximately 6 feet tall with brown hair, police said. She was last seen wearing a red t-shirt with the phrase "Get Over It" printed on the front, ripped blue jeans and tan boots, according to the police press release.

       She may have been carrying a white bag with her belongings inside, police said.

       Anyone with more information regarding Kylee Monteiro's disappearance or whereabouts -- or residents in the area with video surveillance -- are urged to contact the Rehoboth Police Department immediately.