Fixed it for ya!
Well, there are a few scenarios where a sane mother of six might undertake fertility treatments.
One which I’m sure has occurred quite a number of times is a woman whose religious/cultural norms involve very large families and who has needed fertility treatments to conceive all or most of her existing children. For example, an Orthodox Jewish woman in a community where families of 10 or 12 children are the norm, will often use fertility treatments when nature doesn’t do the trick, and if finances permit, might well keep repeating the process throughout her child-bearing years. Certainly kinds of fertility problems (e.g. blocked tubes, and a hormonal variation which only poses a problem for Orthodox Jews — Google “halachic infertility” if you want more on that), pose no obstacle at all to having lots of babies via fertility treatments. If you grew up in a family of 12 children, and your sisters and age-matched neighbor women are all well on their way to similar size broods, and you’ve only got 6, you probably wouldn’t hesitate to pay another visit to the fertility doc.
The other scenario I can think of is a woman whose first six were by one father, and who has remarried and wants at least one child with her new husband.