Pregnancy Week Belly Size Week 1– Week 4 Ovulation takes place. Fertilisation of the egg by the sperm. No noticeable difference in the belly as the baby is about 2mm long during this time. Week 4- Week 8 The baby grows to an inch, but there is unlikely to be any major difference. You may experience some tightness in your belly. Week 8- Week 12 The baby would be about 2.5-3 inches long. You may see an incremental increase in your waist and your pants may get tighter. Week 12- Week 18 Your baby will be about 5 inches...