Stomach of pregnant women who continue to grow along with the gestational age would make the navel that the stomach should be flat with even prominent. Although somewhat disturbing appearance, in fact this is normal in pregnancy.Fetal growth, enlargement of uterine size and increasing the amount of amniotic fluid will cause the stomach enlarges rapidly during pregnancy. These pressures can make the skin tissue widened so that the navel look outstanding.
In a normal body weight gain, skin tissue widening will occur in stages so there is no great pressure that drives the navel. But you do not have to worry about a few months after delivery the navel will return to normal conditions.
source: (http://health.kompas.com/read/2011/03/23/1237530/Mengapa.Pusar.Bumil.Tampak.Menonjol.)