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Thai Girls are renowned for pulling all types of scams and the fake pregnancy is one of them! For Thai girls involved with foreigners claiming they are pregnant is one of the top 10 scams in Thailand. She will say she is pregnant but usually she is not and use this to ask you for your money, either for an abortion or money to raise the baby. Often this trick is played with more than one foreigner at the same time.
Even if she is pregnant it may not even be yours. I would personally go with her to the hospital to do an ultrasound and I would not believe her if she did it by herself as she could use the results from someone else. In case of any doubt if the baby is yours or not I would NOT accept any moral or financial responsibilities untill a DNA test proofs that you are indeed the father.