During pregnancy, the belly increases primarily because the uterus enlarges. At 12 weeks it is the size of a large orange; at 24 weeks it reaches the navel with its tip; at 38 weeks it reaches the costal arches. The size of the uterus depends on the size of the fetus or fetuses and the amount of amniotic fluid. The size and shape of the abdomen depends on the elasticity and fitness of the abdominal wall, hormonal levels and the weight of the mother.
In primiparous women, the abdomen is usually more elastic and sharp, in multiparous women it is wider and saggy. A week or two before birth, the stomach drops and the baby's head is located near the pelvic ring. During examinations, the doctor measures the height and width of the abdomen and the height of the fundus of the uterus to check whether the fetus is developing correctly.
There is no difference in shape between a girl and a boy pregnancy, but since boys are slightly larger, the belly may be slightly larger. But pregnancy with twins or triplets is noticeable by the end of the first trimester.