The 4th trimester, a now famous term first coined by Dr. Harvey Karp, paediatrician and author of Happiest Baby on the Block, refers to the first 3 months of a baby’s life after birth. Dr. Karp explains that babies are born too soon; unlike other mammals who are born being able to walk (more or less), human babies, who can’t even lift their heads when they’re born, remain incredibly vulnerable for many, many months.