The second tri would be the best time to go.

I'd probably avoid Thailand though. They are having political issues there at the moment and it would be awful for you to get stuck there whilst you are pregnant. The only time I've ever had food poisoning was in Thailand from one sip of orange juice which had been watered down with the local water. I was sick for days and thought I was going to die. I've travelled extensively and am extremely careful with food and drink & it still happened to me. Due to both those reasons I'd give Thailand a miss.

Personally I'd be thinking more along the lines of a not so adventurous holiday. One where the food is more like home (cos you can be very fussy about food all the way through) and where the water is safe to drink.