1 of 40 weeks
First trimester
→ Week 1
Week one begins with your last period — pregnancy is counted from here.
Baby size this week
As lovely as a poppy seed
Measurements are averages — every baby grows at their own pace.
Length 0.1 cm 0.0 in
Weight 0.1 g 0.00 oz
Your baby this week
- The first week of pregnancy is counted from the first day of your last period
- Your uterus is building a thick lining to nourish a pregnancy
- Eggs are maturing in your ovaries
- You aren't actually pregnant yet — but the clock starts here
Your body this week
Mild cramping
Bloating
Fatigue
This week's to-do
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How we measure week 1
Pregnancy weeks are counted from the first day of your last menstrual period, following standard ACOG and NHS guidance. Sizes are crown-rump length in early weeks and head-to-heel from around week 20. Your care team may adjust dates after an ultrasound.
BumpCount offers general information and does not replace advice from your doctor or midwife.