First trimester
Week 1 of Pregnancy
Counting begins
Your baby this week
- There is no baby yet — week 1 begins on the first day of your last menstrual period.
- Care teams count from this date because it is reliable and easy to pinpoint, even though conception happens about two weeks later.
- This convention, called gestational age, is used around the world for every due date you will ever be given.
What you may be feeling
- Menstrual flow and cramping
- Bloating
- Lower back ache
Gentle tips for this week
- Mark the first day of your period — this single date anchors every calculation to come.
- If you are trying to conceive, start a prenatal vitamin with at least 400 mcg of folic acid now.
Common question for week 1
Why does pregnancy week 1 start with a period?
Because pregnancies are dated from the first day of the last menstrual period (LMP), not from conception. Conception usually happens about two weeks after LMP, so you are already considered about four weeks pregnant when a test turns positive.
Every pregnancy is different. Sizes are averages, symptoms vary widely, and none of this replaces the guidance of your own provider.