Definition of «first heartbeat»

The phrase "first heartbeat" refers to the very first time a person's heart starts to beat inside their mother's womb.

Snuggled into me and listening to my pulse, just as you had from your very first heartbeat. (attachmentparenting.org)
at the first heartbeat? (rachelheldevans.com)
Medical science establishes that the first HEARTBEAT occurs somewhere during the first 21 to 28 days. (religion.blogs.cnn.com)
(see all sentences)