The tv series is based on the memoirs of Jennifer Worth (Nurse Midwife Jenny Lee from the first few seasons). Most of the storylines from the first few seasons were taken almost directly from those books.
I’ve watched the series from the very beginning (waaaaaay back in 2012) and, while I still love it, I feel like the quality of the writing has gone down. And I also don’t like how sometimes the reasons for characters departures are very unclear and then most of the time said-departed characters are never mentioned again.