"But in the UK (and other international) schools take predictions very seriously: schools work hard to get and keep a reputation for accuracy. University offers are almost all made on predictions, and the universities really don't like having to rescind offers when students don't make their offer."
Actually the're well used to students not getting their predicted grades. They make more offers than they expect places to offset that; and there's also a thing which used to be called Clearing and is now Adjustment in the days after UK A level results come out where those who didn't quite fulfil their offers see where else they can get in at the last minute.
There are some physical advantages to having your kids young - less likely to be complications with pregnancy or birth, less likely to have birth defects, more energy coping with baby/small child. And I'm sure we all know women who put career first with the intention of starting a family later and ended up finding it was too late to have a baby at all, or who only did so after going through the expense and emotional turmoil of IVF.
Another silver lining for your family: the future GGparents will get to see at least one baby from the next generation.
Is D definitely planning to be a SAHM, or is she going to put baby in daycare and continue her career?