Yes, a tourist visa is required. It can be submitted online no more than 90 days prior to arrival. Can fill out form here: https://www.srilankaevisa.lk/
You need a valid passport to travel to Sri Lanka. Your passport must be valid for at least six months from the date you plan to arrive in Sri Lanka.
It is safe to travel to while pregnant. Sri Lanka is free of Zika and malaria, but dengue is present in small numbers. You should take precautions to prevent mosquito bites. Linked below are the CDC's advice and vaccine recommendations. https://wwwnc.cdc.gov/travel/destinations/traveler/none/sri-lanka