Marie Kelly Fauvrelle, 26, was eight months pregnant when she was stabbed in an attack at her home in Croydon, south London.
The baby boy — named by relatives as Riley — remained in hospital clinging to life this morning.
Relatives said family had gathered at his hospital bedside where the child was said to be critically ill.
A 29-year-old man remained in custody being questioned on suspicion of murder today.
Her killing came months after she posted photos of her ultrasound on Facebook alongside the caption:
Happy Valentine’s sweet pea... we can’t wait to meet you.
Ms Fauvrelle was a keen motorcyclist who had last year passed her bus driving licence and had previously worked for Transport for London.
Her brother Stephan Alexander Simpson wrote on Facebook that she was:
A beautiful person whose life has been taken from her. You’ll always be in my thoughts, so please keep watching over us. I love you more than you can possibly imagine. Rest in peace Kelly.
Scotland Yard officers were called to reports of a woman in cardiac arrest at the house just after 3am, where they found Ms Fauvrelle with stab injuries.
She died at the scene. Her child was also delivered at the scene after the stabbing.
After the incident, London Mayor Sadiq Khan said:
Violence against women has no place in our city, and horrific murders in the home like this show the scale of the problem we face. My heart goes out to this innocent child, and to the mother they have so tragically lost. @MetPoliceUK are investigating - please help if you can.