I'd go, but then I went camping (airbed, campsite loos) when I was 37 weeks. Admittedly DH was with me, and was sober enough to drive at all times, and we were with friends who could have looked after DD1 and DD2 if necessary. I went to Egypt at 27 weeks - decided to opt out of camel riding, but could manage everything else.
If 'something happens' at 30 weeks it may well be an emergency, so need an ambulance anyway (and is very unlikely). As already asked - how far are you from the nearest hospital? Is everyone else definitely planning to get drunk? On most hen parties I've been on there have been people not drinking, for a variety of reasons.
Check how 'middle of nowhere' you'll be. Some seemingly remote places have hospitals close by, quicker to get to than getting across a busy town in rush hour.
Could you join them for a day? Have a weekend away with DP or friend, based nearby, where you join the hens just for a day?
Look into it all (which you are doing) - and if, all things considered, you'd spend the hen party, and the weeks before, worrying, it may be best not to go.