What Happens in the First Trimester

Weeks 1-13 at a glance

The first trimester ends at 13 weeks and 6 days. During these weeks your body produces a surge of hormones, the embryo becomes a fetus by about week 8, and by the end of week 13 the fetus is roughly 3 to 4 inches long and weighs about 1 ounce, with fingers, toes, nails, and a beating heart.

You will usually have two to three prenatal visits in the first trimester. The first visit is the longest and often happens around 8 to 10 weeks.

Common first-trimester symptoms

These are typical, and their absence is also normal:

• Nausea or vomiting (so-called morning sickness, which can happen any time of day)
• Fatigue, sometimes severe
• Tender, swollen breasts
• Needing to urinate more often
• Food cravings or aversions and a heightened sense of smell
• Mood changes from shifting hormones

First-trimester care and key tests

Typical timing of early prenatal care. Your provider sets your exact schedule.

Item	When	What it does
First prenatal visit	~8-10 weeks	Health history, bloodwork, Rh factor, dating, due-date estimate
Dating ultrasound	Up to 13 wk 6 days	Most accurate way to confirm gestational age
Cell-free DNA (NIPT) screening	From 10 weeks	Blood test screening for Down syndrome and other chromosome conditions
First-trimester combined screen	11-13 weeks	Blood test plus nuchal translucency ultrasound
Fetal heartbeat by Doppler	~12 weeks	Provider may hear the heartbeat

Folic acid and prenatal vitamins

Folic acid helps prevent neural tube defects, which form in the first few weeks:

• The CDC recommends 400 micrograms (mcg) of folic acid daily for anyone who could become pregnant, ideally starting before conception.
• Most prenatal vitamins include this amount; check the label.
• If you have had a prior pregnancy with a neural tube defect, the CDC recommends 4,000 mcg daily starting at least 1 month before conception through the first trimester. Take this higher dose only under medical guidance.
• Ask your provider before starting or changing any supplement.

What to avoid in the first trimester

• Alcohol, in any amount
• Smoking and vaping
• High-mercury fish such as shark, swordfish, king mackerel, and tilefish
• Raw or undercooked fish, meat, and eggs, and unpasteurized dairy or juice
• Deli meats and hot dogs unless heated until steaming (listeria risk)
• More than about 200 mg of caffeine per day (roughly one 12 oz cup of coffee); confirm your limit with your provider
• Any prescription or over-the-counter medicine not cleared by your provider

Reflects CDC folic-acid guidance and ACOG/Cleveland Clinic/Mayo Clinic prenatal-care guidance, 2024-2026.

Related questions

Is it normal to have no symptoms in the first trimester?

Yes. Symptoms range widely, and some people feel little to no nausea or fatigue. Symptoms can also come and go from day to day. A lack of symptoms by itself is not a sign of a problem, but report any new bleeding or severe pain to your provider.

When should I schedule my first prenatal visit?

Call your provider as soon as you have a positive pregnancy test. The first visit is usually scheduled around 8 to 10 weeks, though your provider may see you sooner if you have bleeding, pain, or a high-risk history.

How accurate is a first-trimester dating ultrasound?

A first-trimester ultrasound performed up to 13 weeks 6 days is the most accurate way to confirm gestational age. It is more precise than dating based on your last period, especially if your cycles are irregular.

What is the difference between NIPT and the first-trimester combined screen?

NIPT (cell-free DNA) is a blood test done from 10 weeks that screens for common chromosome conditions. The first-trimester combined screen, done at 11 to 13 weeks, pairs a blood test with a nuchal translucency ultrasound. Both are screening tests, not diagnoses; abnormal results are followed by diagnostic testing.

How much caffeine is safe in early pregnancy?

Most guidance keeps caffeine under about 200 mg per day, roughly one 12-ounce cup of coffee. Remember that tea, soda, energy drinks, and chocolate also contain caffeine. Confirm your own limit with your provider.

Sources & further reading

1. Cleveland Clinic — First Trimester of Pregnancy: What To Expect
2. CDC — Folic Acid: Sources and Recommended Intake
3. CDC — About Neural Tube Defects
4. ACOG — Routine Tests During Pregnancy (FAQ)
5. Mayo Clinic — Fetal development: The first trimester

This article reflects current AAP, CDC, FDA, and other public-health guidance and is for educational purposes only. It does not constitute medical advice. ParentFlow is a wellness companion — not a substitute for your pediatrician. For any medical concern, contact your healthcare provider.