Start counting from the first day of your last menstrual period
Add 7 days to complete each pregnancy week
Week 1 begins on day 1 of your last period
Week 2 begins on day 8
Week 3 begins on day 15
Week 4 begins on day 22
Continue adding 7 days for each new week
Use the formula: pregnancy week = current date minus first day of last menstrual period
Convert the total days into weeks by dividing by 7
Count any remaining days as extra days in the current week
Example: 35 days since last period equals 5 weeks pregnant
Use an ultrasound date if your doctor gives a more accurate gestational age
Use your estimated due date to track how many weeks remain
Pregnancy usually lasts about 40 weeks from the first day of the last menstrual period