This situation arose in my husband's family this year and it caused a few dramas at the time, but it's more or less ok now. Just wanted to mention how it can feel from the bride to be's perspective though. In my husband's family, one sister in law announced that she was pregnant and not yet 12 weeks (think she was about 8 weeks) and the baby would be due in July/August time. They mentioned it early on because they were starting to tell people face to face at family gatherings as & when they saw them. On the same night, my sister-in-law to be announced that they had been wedding venue browsing and had booked and paid a deposit on their wedding venue and it would be in July (approx 2-3 week window before the baby's due date).
Discussions then ensued with one side of the family wanting to see if they could possibly move their wedding date to accommodate the other sister-in-law's pregnancy, as she would either be a couple of weeks away from giving birth at the wedding (involving a 4.5 hour drive away) or might have had the baby early and would be recovering from a planned c-section and couldn't come and nor could the groom's brother. The bride to be was a bit upset to say the least, as they'd paid out money and secured their dream venue, and didn't know at the time of booking that there was a baby on the way in the family. They were sympathetic but didn't want to consider moving the date as it was a lovely venue for a Summer wedding and moving it earlier or later wasn't part of their wedding plans - financially or in their ideal wedding dreams etc. The fact that the happy couple to be were asked by their family to potentially change the date, upset them and took the wind out of their sails, as what should have been a happy occasion celebrating the fact they had a wedding date, was a bit overshadowed by comments along the lines of: 'but what about xx and xx who probably can't be there now - if only you'd consulted them before booking etc.'
What it meant was that there was a big question mark in the months leading up to the wedding as to whether the groom's brother and wife would be there or not. In the end, they were able to make it to the wedding and the baby was born about 2 weeks after the wedding. It all worked out fine, but was possibly a bit tiring for my sister in law who was pregnant and had to pack the overnight hospital bag incase she went into labour early and was a long way from home. I'm not sure how this situation would've played out if the baby had come a month early and would then be the same sort of timescales that you're looking at of the wedding being planned for post-baby.
Just wanted to say that there is a risk that asking the couple to move their wedding date might be upsetting to the happy couple. I'm sure a lot depends on family relations and flexibility of the venue, how well you all get on etc. I just think it's the sort of thing that can upset the bride to be quite a lot. Not an easy situation, so I'm not judging your ideas in anyway! Just thought I'd mention that this did all work out ok in the end in my husband's family, but definitely put a downer on the happy couple's anticipations of their wedding. So do tread carefully with the bride's feelings.