Educational content, not medical advice. Clinical review of this page is in progress.
The first trimester covers weeks 1 through 13 and is when your body undergoes the most dramatic hormonal shifts of pregnancy, even though there’s little visible change yet.
Common Symptoms
- Morning sickness (nausea, with or without vomiting)
- Fatigue
- Breast tenderness
- Frequent urination
- Food aversions or cravings
Key Appointments and Screenings
Your first prenatal visit usually happens around 8 weeks and includes bloodwork, a dating ultrasound, and a review of your health history. First-trimester screening (around 11–13 weeks) can assess chromosomal risk.
Fetal Development Highlights
By the end of the first trimester, all major organs have begun forming, the heartbeat is detectable, and the baby (now called a fetus) is about the size of a lemon.
What to Expect Week by Week
The first trimester runs from conception through week 13. The embryo implants, the placenta forms, and by week 13 the major organs have begun to develop. Common experiences include fatigue, breast tenderness, nausea and frequent urination. See 10 weeks pregnant and 13 weeks pregnant.
Prenatal Care Basics
Schedule a first prenatal visit as soon as you have a positive test. Ask about folic acid, medications, vaccinations and screening. Learn about testing in Prenatal Screening Tests.
Easing Common Symptoms
Small frequent meals, ginger and rest can help; see morning sickness guide and pregnancy fatigue guide. NHS: Vomiting and morning sickness has practical advice.
Warning Signs
Call your provider or seek urgent care for heavy bleeding, severe one-sided abdominal pain, fainting, or fever. See ACOG: Early Pregnancy, What to Expect.
How the First Trimester Is Counted
Doctors count pregnancy from the first day of your last menstrual period (LMP), not from conception. That means you are already “2 weeks pregnant” on the day you ovulate, and most people only learn they are pregnant around week 4 or 5. The first trimester runs from week 1 to the end of week 13, roughly three months. If your cycles are irregular or you conceived through IVF, your due date may be set differently, so use our due date calculator or IVF due date calculator to see your timeline, and read how doctors calculate gestational age to understand any later adjustments.
Week-by-Week Snapshot
Weeks 1–4: Ovulation, conception and implantation
Weeks 1 and 2 are your period and the days leading to ovulation. Fertilization usually happens within about 24 hours of ovulation, and the embryo implants in the uterine lining roughly 6 to 12 days later. Some people notice light spotting or mild cramping, which is explained in our guides on implantation symptoms and timing and implantation bleeding vs a period. The hormone hCG begins to rise after implantation, which is what pregnancy tests detect (see implantation and hCG).
Weeks 5–8: The embryo period
This is when the brain, spinal cord, heart and major organs begin forming. A heartbeat can often be seen on a transvaginal ultrasound around week 6. Nausea, breast tenderness, fatigue and frequent urination often begin or intensify in this window.
Weeks 9–13: The fetal period begins
By week 10 the embryo is called a fetus. Fingers and toes separate, the face takes shape and the baby starts making small movements you cannot feel yet. For many people, nausea and exhaustion start to ease toward the end of week 12 or 13 as the placenta takes over hormone production.
Prenatal Care Checklist for Weeks 1–13
- Start a prenatal vitamin with folic acid. Major health bodies recommend at least 400 micrograms of folic acid daily before conception and in early pregnancy to lower the risk of neural tube defects. Your provider may recommend a different dose based on your history.
- Book your first prenatal visit. Many practices see you around weeks 8 to 10. Expect a health history review, blood tests, blood pressure and often a dating ultrasound.
- Review your medications. Do not stop prescribed medicine on your own, but ask your provider which prescriptions and over-the-counter products are safe.
- Stop alcohol, smoking and recreational drugs. There is no known safe amount of alcohol in pregnancy.
- Keep caffeine moderate. Many guidelines suggest staying under about 200 mg a day, roughly one 12-ounce cup of coffee.
- Adjust your food choices. Avoid raw or undercooked meat, fish and eggs, unpasteurized dairy and juice, and limit high-mercury fish. Heat deli meats until steaming hot.
- Ask about optional screening. Cell-free DNA screening can usually be done from about 10 weeks, the nuchal translucency scan is typically offered between 11 and 14 weeks, and chorionic villus sampling (CVS) is an optional diagnostic test usually done between 10 and 13 weeks. Our prenatal screening tests guide explains each option.
Managing Nausea, Fatigue and Other Early Symptoms
Nausea is common and often peaks around weeks 8 to 10. Eating small, frequent meals, keeping plain crackers by the bed, sipping fluids through the day, and trying ginger or vitamin B6 (with your provider’s approval) help many people. See our full guides on morning sickness relief and the causes and timeline of morning sickness. For exhaustion, read the pregnancy fatigue guide. Frequent urination and breast tenderness are driven by hormones and blood volume changes and usually settle later in the trimester.
Is It Normal? Spotting, Cramping and Miscarriage Risk
Light spotting and mild cramping affect a significant number of healthy pregnancies, but they can also signal a problem, so they are always worth mentioning to your provider. Early pregnancy loss is common: an estimated 10 to 20 percent of known pregnancies end in miscarriage, most often because of chromosomal changes that nobody could have prevented. Exercise, sex and everyday stress do not cause miscarriage. You can explore general statistics with our miscarriage risk calculator, though no tool can predict an individual outcome.
When to Call Your Doctor Right Away
Contact your provider or seek urgent care if you have any of the following:
- Heavy vaginal bleeding (soaking a pad an hour) or passing tissue
- Severe one-sided abdominal or pelvic pain, shoulder pain, dizziness or fainting, which can signal an ectopic pregnancy
- Fever of 100.4°F (38°C) or higher
- Vomiting so severe you cannot keep fluids down for 24 hours, signs of dehydration, or rapid weight loss, which can indicate hyperemesis gravidarum
- Burning urination or foul-smelling discharge (see the pregnancy discharge guide)
First Trimester FAQs
When should I take a pregnancy test?
Most home tests are reliable from the first day of a missed period. Testing earlier can give a false negative because hCG may still be too low.
When is my first ultrasound?
Many providers schedule a dating scan between 8 and 12 weeks. An earlier scan may be offered if you have pain, bleeding or uncertain dates.
How much weight should I gain?
Most people gain only 1 to 4 pounds in the whole first trimester. Your ideal total depends on your starting BMI, which you can check with our pregnancy BMI calculator.
Can I exercise?
Most healthy pregnancies allow moderate activity such as walking, swimming and prenatal yoga. Ask your provider about any restrictions.
Why do I feel so tired?
Rising progesterone, increased blood volume and the metabolic work of building a placenta all drain energy, and it usually improves in the second trimester.
Next Step: Your Second Trimester
Once you pass week 13, most early symptoms ease. Continue with the second trimester guide, or keep following your progress in pregnancy week by week and pregnancy symptoms by week.
Related Reading
- Second Trimester Guide — weeks 14 to 27
- Third Trimester Guide — weeks 28 to birth
- 10 Weeks Pregnant — baby development and symptoms
- 13 Weeks Pregnant — end of the first trimester
Also see our calculator FAQs, pregnancy FAQs and pregnancy terms glossary.
Related Resources
- Due Date Calculator
- Second Trimester Guide (Weeks 14–27)
- Pregnancy Symptoms by Week
- Morning Sickness Guide: Causes and Relief
- The Complete Pregnancy Fatigue Guide
Sources and Further Reading
This page is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always consult your OB-GYN or midwife about your own pregnancy.