I live in Sri Lanka at the mo, and would say that it's safe for a pregnant woman. It may not be comfortable though with the humidity.
Malaria is relatviely low risk (depends on which areas you are travelling to) but dengue is a very real risk, and I would advise some form of repellent. I've got a young DS and I use citronella spray on him, and tiger balm patches (stick them on his clothes and he's good to go). You could look into that sort of thing.
In terms of food and drink, it's a pretty clean country (compared to India, rather than compared to UK), and I hear very little of food poisoning, and again if you're sensible it shouldn't be a problem.
Just typed all this then realised your original question was back in Feb, so you've probably booked for somewhere else now!!
You need to check with the airline that they are Ok (sure they will be) and with travel insurance because they may not be. also go and see your gp. tbh this is not a trip i would make at 26 weeks pregnant unless i had no choice. anti malarials wont be possible and although deet is safe wearing it all over 100% of the time is not my idea of good when pregnant. if this is a trip by choice could you not look at somewhere else?