While every pregnancy is different, every expecting mom experiences the same pregnancy timeline. A pregnancy is calculated from the first day of the woman’s last period. That means, for the first several weeks, an expectant mom doesn’t know her pregnancy timeline has even begun!

The average pregnancy length is 38-42 weeks. A woman’s first pregnancy lasts an average 274 days from conception or 288 days from the last menstrual period. Babies born before 37 weeks are consider to be preterm and those born after 42 weeks are post-term. By the time the pregnancy timeline comes to an end, your baby probably weighs a little over 7 pounds at this point and measures a little longer than 20 inches, head to heel.

The pregnancy timeline may not end exactly on schedule. Only 3.6% of all women deliver on the due date predicted by last menstrual period and 4.7% give birth on the day predicted by ultrasound. Your baby’s actual birthday can be anywhere from two weeks before to two weeks after your due date.