As I understand it, recent discrimination laws means that insurers have to cover you throughout pregnancy (technically right to the end, but keep reading...)
Pregnancy itself is not considered to be an illness or pre-existing medical 'condition'. Most insurance therefore covers you for complications e.g. HG, pre-eclampsia, miscarriage etc. or anything that is not considered to be routine pregnancy and childbirth.
Typical things you wouldn't be covered for are an airline not letting you fly if its within the terms of booking that they won't carry you at certain weeks (unless you have an annual policy, and booked before knowing you were pregnant) or, say, childbirth when you are considered full term.
I'm with Aviva who make no mention of it at all on our annual policy so called to check and they confirmed the above (we are in China from week 14-17). I also hear that the post office is very comprehensive. Cancellation seems to be the main area where insurers differ, there was a well publicised case recently of a woman who cancelled due to malaria risk which her insurers refused to pay out on. Aviva and post office confirmed they would have.
One thing to bear in mind if you're going to travel late on in pregnancy is whether, if you go.into early labour, whilst you will be covered, what about your baby?
Hope you find a good policy - some of the big insurers can.actually be better than you'd think and cover this as standard without needing a specific pregnancy policy.
Congratulations and have a good time!