The boyfriend of a young mother who died in their Kingswood home is in police custody after neighbours discovered the critically injured woman on the floor.
They tried to save Hayley McClenahan-Ernst, a pregnant mother of two, but it was too late.
Her death is being treated as suspicious.
Her 21-year-old boyfriend Max Spencer was there along with McClenahan-Ernst's cousin Ryan and his partner.
Mr Spencer ran to neighbours and begged them to call an ambulance just after midnight.
Neighbour Ashley Egberts was asked by 7 News how he was, she responded, "Upset, a little erratic but in that circumstance I guess you would be."
"Just surreal, it's not something you expect to happen next door, that's for sure," she said.
Neighbour Kevin Keys found Hayley unconscious in the lounge room and performed CPR.
"She was out cold, yeah in a bad way" he told 7 News.
Paramedics arrived and took over but she couldn't be saved.
Family members rushed to the house but they were told the tragic news.
Investigators say there were no obvious injuries and no blood.
Superintendent Greg Peters from St Marys Police said, "The cause of death as I say is unknown at this point in time."
Mr Spencer was taken away for questioning.
The young couple had been out for dinner earlier in the night. When Mr Spencer ran next door for help, he told neighbours his pregnant girlfriend had fallen and hit her head on the coffee table.
Family and friends remembered the young mother online.
No charges have been laid in relation to Ms McClenahan-Ernst's death.
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