There are a number of changes that happen during pregnancy, thd most relevant to this post are the release of relaxin and the separation of your abs.
Relaxin is a hormone that allows the normally fused pelvic muscles to open and flex to make it possible for a baby to come out. The complication is that all your other joints are also relaxed, which makes them more prone to injury.
The separation of the abdominals is obvious, and the consequence is that your internal organs and spine are no longer supported adequately. This is not a problem while you have a belly full of fluid, but oncd the baby comes out...
The most common problems i see in post natal women could have been easily prevented by adequate preparation. Proper strength training when pregnant means that when your baby comes, and you have less organ and spine muscle support, plus having to lift a 3-5kg baby 24/7, you will be better protected.
It does not make sense to enter into a physically demanding job (which childcare is) "cold turkey".
Five exercises i would ask a pregant women to perform every second day:
1) WALK- great for posture, well being, and pelvic floor
2) Seated row- this one is again for your posture, as well as your back and lifting muscles.
3) Upright row- as above but with greater emphasis on the shoulderx.
4) Hip flexor stretch- flexible hip flexors will make it easier for your abs to pull back together.
5) Clench release- your abs, pelvic floor, internal abs. Breathe through it, and don't stress about squeezing the baby, they are so well protected that it is not a concern.