Pregnancy Due Date Calculator but always ended up delivering around the date calculated by my lmp. the body is not on a clock counting down to delivery; natural variation will introduce uncertainty. an ultrasound date in the first trimester differs from your lmp date by. average time between menstrual periods is about 28 days, and ovulation occurs about 14 days before the next period begins. people discuss how far along in pregnancy someone is, they tend to do so in general terms: “she’s in the first trimester” or “she’s four months along.