First trimester · weeks 1–13 guide

First trimester

Weeks 1 through 13

The first trimester spans weeks 1–13 — conception, implantation, and the formation of all major organ systems. Nausea and deep fatigue are common; folic acid and the first-trimester screen are the key milestones.

Key milestones

  • Conception around week 3; implantation in week 4 — hCG begins to rise.
  • Neural tube forms in weeks 5–6 — folic acid matters most here.
  • Fetal heartbeat visible on ultrasound at week 6 (100–160 bpm).
  • Nuchal translucency (NT) screening at weeks 11–13+6.
  • By the end of week 13: ~7.4 cm, ~23 g. All major organ systems are in place.

Common symptoms and changes

  • Morning sickness peaks at weeks 6–9 and usually fades after week 12.
  • Extreme fatigue and sleepiness.
  • Breast tenderness, frequent urination, heightened sense of smell.
  • Light implantation spotting (pink/brown) may occur — usually benign if brief.
  • Food aversions and cravings.

Recommended prenatal care

  • First prenatal visit at 6–8 weeks — confirm gestational sac and heartbeat.
  • Initial blood panel — rubella, hepatitis B, HIV, blood type, CBC.
  • NT ultrasound at 11–13+6 weeks.
  • First-trimester combined screen (blood markers + NT).
  • NIPT (cell-free DNA) is generally available from week 10.

Nutrition and lifestyle tips

  • Take 400–800 µg of folic acid daily through week 12.
  • For nausea: small frequent meals, ginger, vitamin B6, salty crackers before getting up.
  • Limit caffeine to under 200 mg/day (about one cup of coffee).
  • Gentle walking and stretching are encouraged; avoid vigorous workouts and core stress.

What to avoid

  • No alcohol, tobacco, or secondhand smoke (organogenesis window).
  • Disclose pregnancy before starting any medication, especially ibuprofen and acne drugs.
  • Avoid raw fish, unpasteurized dairy, and undercooked meat.
  • Avoid hot tubs, saunas, and high-heat exposure (neural tube risk).
  • Always tell radiology you may be pregnant before X-rays or CT.

Months in this trimester

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Textbook averages. Individual variation is wide and this is not medical advice — confirm with your OB.