First Trimester Guide: Weeks 1–13 of Pregnancy

The first trimester (weeks 1–13) is when your body undergoes some of its most dramatic changes — even though your baby is still smaller than a peach. From the moment of conception, a cascade of hormonal shifts begins that can bring nausea, exhaustion, and a whirlwind of emotions. Understanding what's happening week by week helps you feel more in control during this overwhelming time.

This guide covers the physical symptoms, emotional changes, medical milestones, and practical preparations you should know about during the first 13 weeks of pregnancy.

Early Signs & Symptoms

Most people discover they're pregnant between weeks 4–6, when a missed period triggers a home pregnancy test. But your body starts sending signals even earlier. Rising hCG (human chorionic gonadotropin) and progesterone levels cause the classic first trimester symptoms.

The most common early signs include a missed period, breast tenderness and swelling, fatigue that feels different from ordinary tiredness, mild cramping or spotting (implantation bleeding around days 6–12 after conception), frequent urination, and heightened sense of smell. Not everyone experiences all of these — some people have very few symptoms in the first weeks.

Nausea — often called "morning sickness" though it can strike at any hour — typically begins around week 6 and peaks between weeks 8–10. According to the American College of Obstetricians and Gynecologists (ACOG), about 70% of pregnant people experience nausea in the first trimester.

What's Happening Week by Week

During weeks 1–4, the fertilized egg implants in the uterine wall and the placenta begins forming. By week 5, your baby's neural tube (which becomes the brain and spinal cord) is developing, and the heart starts beating. This is why folate is so critical in the earliest weeks — the neural tube closes by day 28.

By weeks 6–8, tiny limb buds appear, facial features begin forming, and your baby grows from the size of a lentil to a kidney bean. The heartbeat is visible on ultrasound. Weeks 9–10 bring the transition from embryo to fetus — all major organs are forming, and fingers and toes are taking shape.

During weeks 11–13, your baby's bones begin to harden, and external genitalia start developing (though it's usually too early to determine sex via ultrasound). By the end of the first trimester, your baby is about 3 inches long — roughly the size of a peach — and all major organ systems are in place.

Prenatal Care & First Appointments

Your first prenatal visit is typically scheduled between weeks 8–10 and is the most comprehensive appointment of your pregnancy. According to the National Institutes of Health (NIH), early prenatal care significantly reduces risks of complications. Expect a full medical history review, blood work (blood type, Rh factor, CBC, STI screening, rubella immunity), urine tests, blood pressure check, and your first ultrasound.

Between weeks 10–13, your provider may offer first trimester screening — a combination of blood tests and an ultrasound measuring the nuchal translucency (fluid at the back of baby's neck) to assess the risk of chromosomal conditions like Down syndrome. Non-invasive prenatal testing (NIPT) is a blood test that can screen for chromosomal abnormalities as early as week 10 with high accuracy.

First Trimester Nutrition

You don't need extra calories in the first trimester — focus on nutrient density rather than quantity. The key nutrients are folate (600mcg daily from food and supplements — found in leafy greens, fortified grains, and lentils), iron (27mg daily — your blood volume is already increasing), calcium (1,000mg daily for baby's developing bones), and DHA omega-3 fatty acids (200–300mg daily for brain development).

If nausea makes eating difficult, focus on small, frequent meals. Bland carbohydrates (crackers, toast, rice) are often easiest to tolerate. Ginger — whether as tea, candies, or supplements — has evidence supporting its use for pregnancy nausea. Vitamin B6 (pyridoxine) at 10–25mg three times daily is the first-line recommendation from ACOG for treating nausea.

Common Concerns

Spotting in the first trimester is common and doesn't always indicate a problem — about 25% of pregnant people experience it, according to the March of Dimes. Light spotting can result from implantation, cervical sensitivity, or normal hormonal changes. However, heavy bleeding with cramping warrants immediate medical attention.

Fatigue during the first trimester is intense because your body is building the placenta — an entirely new organ — while progesterone levels surge. Rest when you can. This exhaustion typically improves dramatically in the second trimester.

Mood swings are normal and driven by hormonal fluctuations. However, persistent sadness, anxiety, or inability to function may indicate prenatal depression, which affects about 10–15% of pregnant people. Talk to your provider if emotions feel overwhelming or unmanageable.

What to Avoid

The first trimester is the most critical period for fetal organ development, making it especially important to avoid certain substances and exposures. Stop alcohol completely — the CDC confirms there is no known safe amount during pregnancy. Limit caffeine to 200mg per day (about one 12oz coffee). Avoid raw or undercooked meat, fish high in mercury (swordfish, king mackerel, tilefish), unpasteurized dairy, deli meats unless heated to steaming, and raw sprouts.

Avoid hot tubs and saunas — core body temperature above 101°F in the first trimester is associated with an increased risk of neural tube defects. Skip cat litter (toxoplasmosis risk) and exposure to harsh chemicals or solvents. Always check with your provider before taking any medication, including over-the-counter drugs and herbal supplements.

Frequently Asked Questions

When should I tell people I'm pregnant?

Most people wait until after the first trimester (12–13 weeks) when the risk of miscarriage drops significantly — from about 10–15% in the first trimester to less than 1% after week 13. However, there's no medical rule — tell people whenever feels right to you. Some prefer early support from close family and friends.

Is it safe to exercise during the first trimester?

Yes — ACOG recommends 150 minutes of moderate-intensity exercise per week throughout pregnancy. If you were active before pregnancy, you can generally continue your routine with modifications. Avoid contact sports, activities with fall risk, and exercising in extreme heat. Walking, swimming, prenatal yoga, and stationary cycling are excellent first trimester options.

How much weight should I gain in the first trimester?

Typically 1–5 pounds total during the first trimester. Some people lose weight due to nausea — this is normal and not harmful as long as you're staying hydrated and taking prenatal vitamins. Weight gain accelerates in the second and third trimesters.

When will morning sickness end?

For most people, nausea peaks between weeks 8–10 and improves dramatically by weeks 12–14. About 10% of pregnant people continue to experience nausea into the second trimester. If you're unable to keep food or liquids down, contact your provider — severe nausea (hyperemesis gravidarum) affects 0.3–3% of pregnancies and may need medical treatment.

Can I travel during the first trimester?

Generally yes, though many people prefer to wait until the second trimester when energy improves and nausea subsides. Stay hydrated, move around during long flights to prevent blood clots, and always carry your prenatal records. Discuss any travel plans with your provider, especially for destinations requiring vaccinations.