Hi OP, I started with the OU at three months pregnant and have continued since. A few thoughts.
Firstly, my pregnancy and birth were straightforward and dd became good at going to bed at seven from about eight weeks (with continuing night wakings to well over a year and 5-6am starts as normal though). You can't predict your experience and it could be much, much harder and all-consuming.
My timing was ok because i could get all the cousework done before the birth, then just had an exam to do at three months. I took six weeks off after the birth, then spent six weeks preparing for the exam. That was possible, though timing everything, with bfing (I expressed, so just as often but more predictable and taking less time), even with DP doing his share and GPs doing some, was tight and a bit stressful. I was glad of the 'summer hols' afterwards, starting teh next year when dd was six months.
This second year has been quite doable with good organisation. I stopped bfing at seven months, so my time was freer then. A series of minor baby illnesses had a quite an impact though - keeping me up at night, so too tired the next evening to work, sometimes wiping out whole weeks.
So much depends on how much 'baby free time' others can make available to you. You cannot look after a young baby and work at the same time. Even if you start work the moment they drop off for a nap, it takes a bit of time to get into what you're doing and, as soon as you really get going, they wake up. Using nap time gets easier later, when they settle into a more predictable pattern.
I'd strongly advise taking at least six weeks off after the birth, so you need to get ahead beforehand. Being able to immerse yourself in the new-baby experience is really important and you'll be exhausted.
I think you'd be fine starting at three months or, better, at six. Working through to six months with only a short break will be hard and will put a lot of pressure on everyone around you. So, while it's not necessary to wait until nine months, the timing of your pg and the academic year is such that I do think you'll find that so much easier than starting this year.
Quite a gap year experience, surely! I can't see why delaying would prevent you from doing it. You'll probably feel very ready by nine months.
The other thing is that, if you have to withdraw from the course part way through (with OU can do it at a third and two thirds of the way through) you get some money back but your work counts for nothing, you'd have to take the same course again for the credit. You can't know how things will go, medically, or what sort of baby you'll get, sleep and demandingness-wise, so that is a possibility to plan and budget for.