At the beginning of the pregnancy it is already determined whether it is a single child or twins; if it is not twins at the beginning, it is usually not possible to become twins after two months. How many children a woman is pregnant with is already very clear at the time of the formation of the fertilized egg; if the uterus contains two fertilized eggs there will be heterozygous twins; if the same fertilized egg splits twice, there will be identical twins, and there will not be any change after this. Therefore, if the ultrasound indicates one gestational sac and one yolk sac at the beginning of the pregnancy, it is clear that the pregnancy is monozygotic; if there are two gestational sacs or two yolk sacs, it is clear that there are twins, and this does not usually change after the second trimester of pregnancy. If you want to know whether you are pregnant with twins, you can make it clear through ultrasound under the guidance of your doctor, and pay attention to avoiding strenuous exercise in the early stages of pregnancy, pay attention to rest, and pay attention to obstetric examination on time.