By that I mean a girl you didn't originally knock up (so not your wife/gf that got pregnant). 

If so how was it? How did you meet her? How long did you guys date? Etc. 

Share whatever you've got, I'm curious to hear!

Two times has this happened to me YEARS ago:

First one was right out of college. Girl I started officially dating had just broken up with her ex a few weeks earlier. She must have had the easiest first trimester ever - no morning sickness, no noticable weight gain, periods were already irregular her whole life. 3 weeks into dating, we finally start to notice her lower belly is getting a little bigger, then find out she was already 12 weeks along! We both decided it was best to end things. She carried small pretty much the whole time until ballooning right up near the end. IIRC, she ultimately got back with that ex.

About 3 years later, but still back when CL was a thing, I spent a month working in another state and happened to connect with someone during my first week there. She was 7 months along and just recently separated from her husband, so we both knew it would be casual and end after the month. Her older children (she was 35) were visiting family in Brazil, so she had her place to herself until they + others would return to stay and help with new baby. She and I saw each other that whole time and it was perfect. I keep telling myself that I'm going to write about it some day.

Met my now-fiancé a year after that!

The appetizer is that a woman I matched with on a dating app showed up to our first date with a very large and unexpected-by-me pregnant belly. There were no indications that she was pregnant before meeting her with her full term bump in-person. It was just one kind of awkward date that didn't last very long.

The main course is an incredible string of good fortune I had in 2015-2017, back when OkCupid let you search by keywords. In succession, I matched with three women when they were early in their pregnancies and enjoyed the privilege of nearly two full years of daily texting/sexting with three attractive pregnant women. As much as I enjoyed the sexting and nudes (and I did), maybe the best part was receiving an exclusive account of their experience with pregnancy. The first two lived too far to realistically visit but the third lived barely close enough for me to drive to her to spend a weekend together when she was late in her third trimester. It was my first up-close, intimate experience with a pregnant woman, and it was absolutely captivating.