Illustration of prenatal development at week 1
Microscopic view, greatly enlarged.

First trimester, week 1

1 week pregnant: A new cycle begins

Size
Nothing yet
Length
— not yet
Weight
—
Heart rate
— not yet
Due date
273 days away 3% of the way
12345678910111213141516171819202122232425262728293031323334353637383940

Checked against the sources listed on this page. Not reviewed by a clinician. Last updated 5 November 2026. How we check.

At 1 week pregnant there is no baby yet. Pregnancy is dated from the first day of your last period, so this is your period week, about two weeks before you conceive.

Your baby this week

  • There is no embryo yet. Pregnancy is dated from the first day of your last period, so week 1 is your period week.
  • Hormones (mainly FSH) wake up a group of follicles in your ovaries. Each follicle holds one immature egg.
  • Only one follicle will usually win the race and release an egg about two weeks from now.

Your body this week

  • Your period sheds last cycle’s uterine lining, and a fresh one starts to build right after.
  • Cramps, tiredness and lower back ache are common and normal.
  • If you are trying to conceive, this is a good moment to review medicines, supplements and habits with a clinician.
Typical weight gain so far
0–0 kg
Your uterus
About the size of a pear (unchanged to the eye)

Weight gain range is for a pre-pregnancy BMI of 18.5–24.9 (IOM guideline). Your provider may advise differently.

Scans, tests and checkups

  • From week 4
    Home pregnancy test Self-care

    Detects hCG in urine. Most reliable from the first day of a missed period.

See the full scan and test timeline

Real size, next to an adult hand

Adult hand 18.5 cmbaby 0.1 mmThe ring marks a stage too small to see at true scale.

Growth curve

01 kg2 kg3 kgwk 10wk 20wk 30wk 40
Typical weight by week: WHO median from week 20, approximate before that. Healthy babies vary widely.

Tip for this week

Start a daily folic acid supplement (400 micrograms) now if you are trying. It cuts the risk of neural tube defects, and the neural tube closes before most people know they are pregnant.

Myth or fact?

Is pregnancy really 9 months?

Partly true

Roughly. It is 40 weeks (280 days) from the first day of your last period, which is about 9 months and 1 week. From conception it is about 38 weeks.

Read the evidence

Questions people ask about week 1

Am I pregnant at week 1?

No. Week 1 is the week of your period. Doctors count pregnancy from the first day of your last period because that date is easier to know than the exact day of conception. Conception usually happens around the end of week 2.

Why is pregnancy 40 weeks if conception happens at week 2?

The 40 weeks are counted from your last period, not from conception. From conception to birth is about 38 weeks.

References

  1. MedlinePlus (US National Library of Medicine): Fetal development
  2. NHS: Vitamins, supplements and nutrition in pregnancy

Advice differs between countries, so follow your own care team’s plan. Weights from week 20 are the WHO median, and earlier weights are approximate. How we check this.

All 40 weeks

First trimester Weeks 1–13

From conception to a fully formed, tiny fetus. The most rapid change of the whole pregnancy.

  1. Prenatal development at week 1 Week 1A new cycle begins
  2. Prenatal development at week 2 Week 2Ovulation — and maybe conception
  3. Prenatal development at week 3 Week 3A ball of cells finds its home
  4. Prenatal development at week 4 Week 4Poppy seed
  5. Prenatal development at week 5 Week 5Sesame seed
  6. Prenatal development at week 6 Week 6Lentil
  7. Prenatal development at week 7 Week 7Blueberry
  8. Prenatal development at week 8 Week 8Raspberry
  9. Prenatal development at week 9 Week 9Cherry
  10. Prenatal development at week 10 Week 10Strawberry
  11. Prenatal development at week 11 Week 11Fig
  12. Prenatal development at week 12 Week 12Lime
  13. Prenatal development at week 13 Week 13Lemon

Second trimester Weeks 14–27

Growth, movement and senses. Often the most comfortable stretch for parents.

  1. Prenatal development at week 14 Week 14Peach
  2. Prenatal development at week 15 Week 15Apple
  3. Prenatal development at week 16 Week 16Avocado
  4. Prenatal development at week 17 Week 17Pear
  5. Prenatal development at week 18 Week 18Bell pepper
  6. Prenatal development at week 19 Week 19Mango
  7. Prenatal development at week 20 Week 20Banana
  8. Prenatal development at week 21 Week 21Carrot
  9. Prenatal development at week 22 Week 22Papaya
  10. Prenatal development at week 23 Week 23Grapefruit
  11. Prenatal development at week 24 Week 24Ear of corn
  12. Prenatal development at week 25 Week 25Cauliflower
  13. Prenatal development at week 26 Week 26Zucchini
  14. Prenatal development at week 27 Week 27Head of lettuce

Third trimester Weeks 28–40

Weight gain, brain and lung maturation, and getting into position for birth.

  1. Prenatal development at week 28 Week 28Eggplant
  2. Prenatal development at week 29 Week 29Butternut squash
  3. Prenatal development at week 30 Week 30Cabbage
  4. Prenatal development at week 31 Week 31Coconut
  5. Prenatal development at week 32 Week 32Napa cabbage
  6. Prenatal development at week 33 Week 33Pineapple
  7. Prenatal development at week 34 Week 34Cantaloupe
  8. Prenatal development at week 35 Week 35Honeydew melon
  9. Prenatal development at week 36 Week 36Papaya (large)
  10. Prenatal development at week 37 Week 37Winter melon
  11. Prenatal development at week 38 Week 38Pumpkin (small)
  12. Prenatal development at week 39 Week 39Mini watermelon
  13. Prenatal development at week 40 Week 40Watermelon