Free family guide · Pregnancy week by week · First trimester

1 to 3 weeks pregnant

ACOG says pregnancy is assumed to start 2 weeks after the first day of the last menstrual period, so the first 2 weeks are counted before you are actually pregnant.[1]

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Sources on this page: ACOG; OWH; USPSTF; NHS; CDC; NIMH; HRSA; MedlinePlus

This page shares what health organizations say. It is not medical advice. It is general information. Talk to your doctor, midwife or pediatrician about your own situation.

In an emergency, call 911.

At a glance

  • ACOG's Committee Opinion 700 says the due date is, by convention, 280 days after the first day of the last period.[2]
  • OWH says a period comes 14 to 16 days after ovulation unless pregnancy occurs.[3]
  • The US Preventive Services Task Force advises a daily folic acid supplement starting at least 1 month before conception.[4]

How weeks 1 to 3 are counted

ACOG says a normal pregnancy lasts about 40 weeks from the first day of the last menstrual period.[1] The NHS also says pregnancy is dated from the first day of your last period.[5]

ACOG's Committee Opinion 700 says the 280-day rule assumes a regular 28-day cycle with ovulation on day 14.[2]

Sources differ

Sources start their week-by-week counts at different points:[6][7][1]

  • OWH counts the first trimester from week 1 to week 12.[6]
  • The NHS week-by-week guide starts at week 4.[7]
  • ACOG counts the first trimester from the first day of the last period to 13 weeks 6 days.[1]

Ovulation and the fertile window

OWH says the average cycle is 28 days, and normal cycles range from 21 to 35 days.[3] It says there are about 6 days in each cycle when you can get pregnant, called the fertile window.[8]

OWH says sperm can survive up to about 3 days.[3]

ACOG says implantation spotting can happen 1 to 2 weeks after fertilization.[9]

Things to do now

  • The US Preventive Services Task Force advises 0.4 to 0.8 mg (400 to 800 mcg) of folic acid a day, starting at least 1 month before conception and continuing through the first 2 to 3 months of pregnancy.[4]
  • CDC says neural tube defects form in the first weeks of pregnancy, often before a person knows they are pregnant.[10]
  • CDC says there is no known safe amount of alcohol during pregnancy and no safe time to drink.[11]
  • CDC says to talk to your providers before you start or stop any medicine.[12]

When a test can work

Sources differ

Sources describe test timing in different ways:[5][13][14]

  • The NHS says some home tests can tell after about 3 and a half weeks.[5]
  • Its week 5 page says tests are sensitive from week 3 or 4 onwards.[13]
  • OWH says results are more reliable about a week after a missed period.[14]

Feelings check-in

The National Institute of Mental Health (NIMH) says perinatal depression includes depression during pregnancy, and its signs last at least 2 weeks.[15]

HRSA says the National Maternal Mental Health Hotline, 1-833-852-6262, takes calls and texts 24/7 in English and Spanish.[16]

When to call your provider or get help

OWH says to call a doctor after a positive home test, or if you get two negative results but still think you are pregnant.[14]

MedlinePlus says early signs of an ectopic pregnancy may include light bleeding and pelvic pain, and that it can be a medical emergency.[17]

Questions families ask

Am I pregnant in week 1?

ACOG says pregnancy is assumed to start 2 weeks after the first day of the last period, so in weeks 1 and 2 you are not yet pregnant.[1]

When can I take a pregnancy test?

OWH says results are more reliable about a week after a missed period.[14] The NHS says some tests can tell after about 3 and a half weeks.[5]

When should I start folic acid?

The US Preventive Services Task Force advises starting at least 1 month before conception.[4]

Free from UpOnly Learning. Written for families of every faith. No outside ads and no affiliate links. Every fact on this page comes from the sources listed below. How we source and review these pages

Updated 2026-10-24: first published.

Sources

All sources were opened and checked on the date shown.

  1. American College of Obstetricians and Gynecologists. How Your Fetus Grows During Pregnancy. Checked October 24, 2026.
  2. American College of Obstetricians and Gynecologists (with AIUM and SMFM). Methods for Estimating the Due Date (Committee Opinion No. 700; reaffirmed 2025). Checked October 24, 2026.
  3. Office on Women's Health (HHS). Trying to conceive. Checked October 24, 2026.
  4. US Preventive Services Task Force. Folic Acid Supplementation to Prevent Neural Tube Defects: Preventive Medication. Checked October 24, 2026.
  5. NHS (United Kingdom). 4 weeks pregnant guide. Checked October 24, 2026.
  6. Office on Women's Health (HHS). Stages of pregnancy. Checked October 24, 2026.
  7. NHS (United Kingdom). Week-by-week guide to pregnancy. Checked October 24, 2026.
  8. Office on Women's Health (HHS). Ovulation calculator. Checked October 24, 2026.
  9. American College of Obstetricians and Gynecologists. Bleeding During Pregnancy. Checked October 24, 2026.
  10. Centers for Disease Control and Prevention. About Folic Acid. Checked October 24, 2026.
  11. Centers for Disease Control and Prevention. About Alcohol Use During Pregnancy. Checked October 24, 2026.
  12. Centers for Disease Control and Prevention. Medicine and Pregnancy: An Overview. Checked October 24, 2026.
  13. NHS (United Kingdom). 5 weeks pregnant guide. Checked October 24, 2026.
  14. Office on Women's Health (HHS). Knowing if you are pregnant. Checked October 24, 2026.
  15. National Institute of Mental Health (NIH). Perinatal Depression. Checked October 24, 2026.
  16. Health Resources and Services Administration (HRSA). National Maternal Mental Health Hotline. Checked October 24, 2026.
  17. MedlinePlus (National Library of Medicine). Ectopic Pregnancy. Checked October 24, 2026.