Muzzy- DH of FreakedOut here. I work in travel insurance claims.
I haven't ever seen a policy that specifically excludes cover for cancellation of a trip due to an assisted pregnancy but many will exclude cover for pregnancy itself (assisted or unassisted) or pregnancy related complications after a certain amount of time. Usually after the third trimester but that depends on the wording of the policy.
The problem that you face is that insurance is designed to cover you for things that are unexpected and beyond your control. And you will agree that a successful assisted pregnancy is not unexpected or beyond your control. If you booked your trip knowing there was a possibility that you could be pregnant and unable to fly then it would be considered in one way to be a preexisting medical condition that will be outright denied.
However, if you happened to have an "unexpected" or "unplanned" pregnancy (as far as your insurance company knows) then you MIGHT have a successful claim.
Some things to think about:
1: when submitting a claim you'll likely have to sign a privacy waiver allowing doctors to release your medical info but the insurance company does not know who your doctors are unless you tell them.
2: your GP (or maybe one you haven't seem before) does not treat you for fertility so a medical cert saying "this patient is unfit to fly due to unexpected pregnancy" will likely be enough for the insurance company. But the insurance company might request copies of all clinical notes so if your GP referred you to a fertility specialist then that could cause an issue.
3: the value of the claim plays a big role in how far they will investigate the details. If its only a few thousand then they're probably not going to camp an investigator outside your house and might not even bother to contact your GP if you give them just enough paperwork. If it's more than $10k then they might do both.
4: there are plenty of unexpected medical conditions that can prevent someone from flying. Gastro, migraines etc which a GP will not deeply investigate but will always write a cert for.
I hope this info has helped and best of luck with your next cycle.