Every woman is different. If you're a first-time mum, you could begin developing a baby bump any time between 12 and 16 weeks. But if this isn't your first baby, you may start showing sooner. At 16 weeks of pregnancy, your uterus will be stretching to fit your growing baby. Your increasing blood and amniotic fluid volume, growing breasts, expanding uterus, and placenta have caused you to gain weight, but you may not be showing.