The early pregnancy begins on the first day of your last menstruation and ends at the end of week 12. Your baby grows quicker in the first trimester than in any other trimester. Use our Due Date Calculator to figure out how far along you are and when you should expect to deliver. This calculator assumes that your periods follow a 28-day cycle. After a dating scan, your doctor will need to confirm your days if your cycles are longer, shorter, or irregular.