One of our lecturers at my old job went from a Bianchi road bike to some mixte frame her husband had built up for her when she got pregnant. I know she was commuting (bi-modal with the train) on that bike until at least 6 months or so.

My wife rode recreationally on her tiny MTB (she is 5'3") until about 6 months into her first pregnancy.

We went with the trailer as it would (and now it does) hold 2 kids and we'd always planned to have 2. It's easier to ride with (lower center of gravity), and easier to get the kids into than a seat. And as mentioned before, there is room back there for a diaper bag and farmer's market purchases. And if you go too crazy, you can still put panniers on the tow bike.