Week 1 of Pregnancy
Not yet pregnant. This is the week of your last menstrual period — clinically counted as week 1 because the actual conception date is rarely known.
Fetal size
Seed stage
- Length
- Not yet measurable
- Weight
- Not yet measurable
Fetal development
- No conception yet. The uterine lining is shedding during menstruation.
- Ovaries begin maturing follicles for the next ovulation.
Maternal changes
- You are menstruating. Gestational age is counted from the first day of the last period (LMP).
- A daily 400–800µg folic acid supplement helps prevent neural tube defects if conception occurs.
Recommended checkups
- If you have had preconception counseling, continue the prenatal vitamins and lifestyle changes your clinician recommended.
Related calculators
- Due date Calculate pregnancy week and due date from your last period
- Pregnancy test timing Find when to test from your last intercourse or ovulation
- βhCG doubling time Calculate hCG doubling time and daily rise from two early pregnancy blood tests
- Cycle Calculate next period, ovulation, and fertile window
Related topics
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Textbook averages. Individual variation is wide and this is not medical advice — confirm with your OB.