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The only sure way to know is via an ultrasound with your doctor or midwife. I was so huge with my first pregnancy that everyone thought it was twins, but there was only one little one in there! Also, sometimes women who are carrying twins notice that their "symptoms" are more severe (e.g. morning sickness is worse/lasts longer than with previous pregnancies; certain first trimester symptoms don't go away at the end of the first trimester; etc.). However, since every pregnancy is different, an ultrasound is the only way to be sure. Good luck!
I am pregnant with twins and the biggest problem I've had is very bad headaches everyday. My OB says that it is quite common for women who are carrying multiples to have them often.