It's up to you and your partner - not your respective friends or family.
I got married when I was eight months pregnant with my first. It was just easier to do - we didn't have to worry about last names or custody stuff or anything. We just eloped pretty much and did a JP ceremony instead of a wedding. We were going to have a real wedding somewhere down the line, but eight years later, that never happened. It kinda sucks in a way, but on the other hand, it doesn't. The marriage is what's important to us, not the actual wedding day. Some friends of ours who had fabulously planned weddings got divorced within a few years.
OTOH I do get wanting to make it a special day, and being pregnant kind of would put a damper on the traditional wedding. You'd have people questioning your commitment - are you JUST getting married for the baby, etc. I never had any real ceremonies in my life - sweet sixteen, prom, high school graduation, college graduation... at this rate I doubt I'll have a funeral, either.
So I can see why a ceremony could be deeply meaningful for you, and you deserve to do it your own way. Being pressured into it by the in-laws is definitely not something that would feel good. I say, do it the way it'll be most special to you - but in the end, it's just a day.
Whether you do it before the baby is born, or after, the marriage is what's most important and lasting. A few years down the line I bet the in-laws won't still be talking about you choosing to delay the ceremony by a few months. (And if they are still talking about it, pshh, then those people have issues that have nothing to do with you!!)