As a woman in her early 30s, is it better to have an adult spinal fusion before or after having kids?
The quick answer is that it is impossible to know!
If she is in extreme pain (which she does not mention), then perhaps it feels urgent.
On the other hand, considering her biological clock is ticking, it makes the most sense for her to have children as soon as possible - as long as she feels physically strong enough to go through pregnancy and the demands of infant care.