See which pregnancy week and trimester you're in right now, based on the first day of your last menstrual period.
Pregnancy is counted from the first day of your last menstrual period, even though conception itself normally happens around two weeks later:
A full-term pregnancy is considered to last 40 weeks (280 days) from this date. The trimesters are divided as follows: 1st trimester runs from week 1 through week 13, 2nd trimester from week 14 through week 27, and 3rd trimester from week 28 until birth.
The exact time of conception is rarely known with certainty, while the date of the last menstrual period is easier to pin down. That's why healthcare providers use this date as the standard starting point for counting pregnancy weeks.
Both use the same calculation method. This calculator's main focus is which week and trimester you're in right now, while a due date calculator's main focus is the calculated birth date itself.
The calculation assumes an average 28-day menstrual cycle. If your cycle deviates a lot from this, a midwife or doctor can adjust the pregnancy week based on an ultrasound measurement early in pregnancy.