First Trimester: Weeks, Symptoms, What to Avoid

The first trimester is the first 12 to 14 weeks of pregnancy, counted from the first day of your last menstrual period, when your baby’s major organs begin to form and nausea and fatigue tend to peak.

These early weeks carry a lot: a positive test, a body that feels different by the day, and a long list of things people say you should or should not do.

This guide stays at trimester level: weeks, development, symptoms, appointments, what to avoid, sleep and when to call. Every clinical detail was read in October 2026 on the primary sources named in the text.

Key Takeaways

The first trimester starts on the first day of your last period and ends between week 12 and week 14, depending on the source.
All major organs begin to form. By 12 weeks the baby is about 3 inches long (Office on Women’s Health).
Tiredness, nausea, sore breasts and frequent urination are common. UPMC says nausea is usually worst around week 9.
The CDC says all women capable of becoming pregnant should get 400 mcg of folic acid daily. Avoid alcohol and smoking.
Heavy bleeding, severe one-sided pelvic pain, shoulder pain or fainting need emergency care right away.

Table of Contents

First Trimester Weeks: When It Starts and Ends

Quick Answer

The first trimester starts on the first day of your last menstrual period and lasts about 12 to 14 weeks, roughly the first three months of pregnancy. Sources end it at different points: the Office on Women’s Health at week 12, Mayo Clinic at about 13 weeks, the American College of Obstetricians and Gynecologists (ACOG) at 13 weeks and 6 days, and MedlinePlus through week 14.

Source	How it defines the first trimester
ACOG, as reported by UPMC	Ends when you have passed 13 weeks and 6 days. The second trimester starts at 14 weeks 0 days.
Cleveland Clinic	Starts on the first day of your last menstrual period and “lasts until the end of the 13th week of pregnancy.”
Mayo Clinic	Describes the first trimester as “about the first 13 weeks.”
MedlinePlus (NIH)	Says it starts at conception and “continues through week 14 of your pregnancy.”
Office on Women’s Health	Week 1 through week 12.

When does the first trimester end? Definitions differ by source, so all five are printed.

How long is the first trimester? About three months. The shortest and longest definitions are two weeks apart, and your care does not change on the day you cross the line.

How the weeks are counted. Pregnancy is counted as about 40 weeks from the first day of your last normal period (Office on Women’s Health). UPMC says doctors use that date for simplicity and that conception happens around week 3, so the first trimester of pregnancy begins before you are actually pregnant. MedlinePlus says a first trimester ultrasound gives an idea of your due date, so your dates may shift.

Want one week at a time? See 4 weeks pregnant, 5 weeks pregnant and 13 weeks pregnant.

How Your Baby Develops, Weeks 1 to 13

Months are a counting convention, not a medical unit. This table uses four-week months.

Weeks	Month	Development highlights	Size, by source
1 to 4	1	Conception happens around week 3 (UPMC). The structures that become the amniotic sac, placenta and umbilical cord start to form, along with a tube that becomes the brain and spinal cord (Cleveland Clinic).	UPMC: about the size of a poppy seed at week 4. Office on Women’s Health: 1/25 inch at 4 to 5 weeks.
5 to 8	2	In the sixth week the heart, lungs, ears, arms and legs begin to form (Cleveland Clinic). By 8 weeks all major organs have begun to form, the heart beats with a regular rhythm, and fingers and toes are forming (Office on Women’s Health).	Office on Women’s Health: nearly 1 inch at 8 weeks. Cleveland Clinic: about 1 inch, roughly the size of a raspberry.
9 to 13	3	The embryo becomes a fetus by week 10 (UPMC). Toes, fingers and nails appear, and the urinary and digestive systems work (Cleveland Clinic). At 12 weeks the baby can make a fist and the eyelids close (Office on Women’s Health).	Office on Women’s Health: about 3 inches and almost an ounce at 12 weeks. Cleveland Clinic: 3 to 4 inches, about the size of a plum.

First trimester development by week range. Size estimates differ by source.

For your own changing shape, see pregnant belly by month. For the hormone behind a positive test, see hCG levels by week.

Symptoms in the First 13 Weeks and What Helps

The Office on Women’s Health says hormonal changes affect almost every organ system in these weeks. It also says some women feel no discomfort at all. Common first trimester symptoms:

Symptom	What the sources say	What can help
Nausea (morning sickness)	Can come at any time of day. UPMC says it is usually worst around week 9, then often goes away in the second trimester.	Smaller meals more often (UPMC). A few crackers before bed if nausea keeps you up (MedlinePlus).
Fatigue	Extreme tiredness (Office on Women’s Health). UPMC says it most commonly peaks around week 7 or 8.	An afternoon nap and short walks outdoors (UPMC). Going to bed earlier (Office on Women’s Health).
Breast changes	Tender, swollen breasts (Office on Women’s Health) as they grow and prepare for breastfeeding (UPMC).	Mention pain that worries you at your visit.
Frequent urination	Picks up toward the end of the first trimester (UPMC).	Limiting caffeine can help, but keep drinking water (UPMC).
Cravings and aversions	Cravings or distaste for certain foods (Office on Women’s Health).	Tell your doctor if you crave nonfoods such as dirt or ice (UPMC).
Constipation	Slower digestion, plus iron in prenatal vitamins (UPMC).	More water and more fiber (UPMC).
Mood swings	Due to hormonal changes (UPMC).	Talking to a friend or a therapist, yoga or a walk (UPMC).

First trimester symptoms and what helps, by source.

Headache, heartburn, skin changes and mild shortness of breath are also listed. Most of these discomforts go away as pregnancy progresses (Office on Women’s Health).

Your First Prenatal Visits and Early Pregnancy Tests

Call to book as soon as you have a positive test. UPMC says the first prenatal visit usually happens around 8 weeks and lasts about 45 minutes. MedlinePlus says you will then have a visit every month in the first trimester.

Exam and history: a pelvic exam with a Pap test, and questions about your health history and medicines (MedlinePlus).
Blood and urine tests: MedlinePlus lists a complete blood count, blood typing with an Rh screen, rubella, hepatitis, syphilis and HIV tests, a cystic fibrosis screen, and a urine analysis and culture.
Dating ultrasound: MedlinePlus says you should have an ultrasound in the first trimester to get an idea of your due date. UPMC places it near the second visit, around weeks 11 to 13, while Cleveland Clinic says the first visit may include one.
Genetic screening: MedlinePlus says all women are offered it. UPMC describes two options offered around weeks 11 to 13: the first-trimester screen (a blood test plus an ultrasound measuring fluid at the back of the baby’s neck) and the noninvasive prenatal test, or NIPT (a blood test).

Screening tests assess risk (UPMC), so ask what a result would mean and what the next step would be.

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Folic Acid, Food and Weight Gain

Folic acid: the CDC states: “All women capable of becoming pregnant should get 400 micrograms (mcg) of folic acid daily.” Neural tube defects, which are major birth defects of the brain and spine, happen in the first few weeks of pregnancy, often before you know you are pregnant.
Prenatal vitamin: UPMC says to take one with folic acid daily. Choose it with your provider.
Fish: up to 12 ounces a week of cooked fish and shellfish with little or no mercury, such as shrimp, salmon, cod and canned light tuna (Office on Women’s Health).
Weight gain: the Office on Women’s Health lists weight gain or loss as a first trimester change, and says most women need 300 calories a day more during at least the last six months of pregnancy. Recommended totals for the whole pregnancy depend on your weight before pregnancy, so ask your provider what range fits you.

What to Avoid in the First Trimester

Alcohol: the Office on Women’s Health: “There is no known safe amount of alcohol a woman can drink while pregnant.”
Smoking: it raises the risk of low birth weight and miscarriage (Office on Women’s Health). Ask your doctor about ways to help you quit.
Too much caffeine: March of Dimes: “If you’re pregnant, limit caffeine to 200 milligrams each day.” That is about one 12-ounce cup of coffee. Tea, some soft drinks, energy drinks and chocolate count too.
Risky foods: the Office on Women’s Health says do not eat refrigerated smoked seafood; hot dogs or deli meats unless steaming hot; unpasteurized milk, juices or soft cheeses; store-made chicken, egg or tuna salad; raw sprouts; or shark, swordfish, king mackerel and tilefish. Cleveland Clinic adds raw fish and undercooked meats.
Recreational drugs, hot tubs, saunas and contact sports: all on Cleveland Clinic’s list of things to avoid.
Unreviewed medicines and herbs: the Office on Women’s Health: “Always speak with your doctor before you start or stop any medicine.” That includes over-the-counter and herbal products.

Exercise, Sleep and Fatigue

Exercise. The Office on Women’s Health says healthy pregnant women should get at least 2 hours and 30 minutes of moderate-intensity aerobic activity a week. UPMC says light to moderate exercise is safe during pregnancy, especially if you were already exercising.

Sleep. MedlinePlus says you may sleep well during the first trimester and may also need more sleep than usual. Its tips:

Go to bed and wake up at the same time each day, and take short naps.
Limit soda, coffee and tea. March of Dimes notes that caffeine may give you trouble sleeping.
Avoid a lot of fluids or a big meal within a few hours of bed.
Do not take any sleep aids, including over-the-counter medicines and herbal products. See can you take melatonin while pregnant.

MedlinePlus’s position advice (side sleeping, and not flat on your back) is not tied to a specific week. We cover it in our third trimester guide. Foggy as well as tired? See pregnancy brain.

Miscarriage: A Gentle Look at the Facts

It is normal to think about this in the early weeks. Mayo Clinic defines miscarriage as the loss of a pregnancy before the 20th week and says about 10% to 20% of known pregnancies end in miscarriage. Most happen during the first trimester.

Mayo Clinic is clear about cause. About half to two-thirds of first trimester miscarriages are linked with extra or missing chromosomes, and most of the time miscarriage is a random event that is no one’s fault. It is not caused by exercise (as long as you are healthy), sex, an argument, or working, unless you are exposed to high doses of harmful chemicals or radiation.

Mayo Clinic’s symptom list includes bleeding with or without pain, including light spotting, and pain or cramping in the pelvic area or lower back. Always report bleeding.

When and How to Tell People

There is no medical rule. Some people share the news right away because they want support whatever happens. Others wait until the end of the first trimester, since most miscarriages happen in these weeks. Both choices are reasonable. Tell your employer sooner if your job involves chemicals, radiation or heavy physical work.

Ready to share? See our pregnancy announcement ideas. Found out later than you expected? Read about cryptic pregnancy.

Your To-Do Checklist for the First 13 Weeks

Book your first prenatal visit and write down the first day of your last period.
Take folic acid daily (the CDC figure is 400 mcg) and choose a prenatal vitamin with your provider.
Stop alcohol and smoking, and ask for help quitting if you need it.
Bring every medicine, supplement and herbal product you use to your first visit.
Learn the food list above and keep caffeine to the March of Dimes limit.

Related Betteroo Guides

When to Call Your Provider

Get emergency care right away. Mayo Clinic says to contact your pregnancy care team or go to an emergency department right away if you have heavy bleeding, severe pain on one side of your pelvis, shoulder pain or fainting. These are warning signs of an ectopic pregnancy, which needs immediate treatment. If you cannot get there safely, call 911.

Call your doctor or midwife for any of these. UPMC adds that if your symptoms are serious, you should go to the emergency department:

Severe nausea and vomiting: too much vomiting may make it hard to keep down fluids (UPMC).
Vaginal bleeding, spotting or cramping: UPMC says these can be normal in the first trimester but can also be a sign of a miscarriage or an ectopic pregnancy.
Fever: a temperature above 100.4 F (UPMC), or fever with chills (MedlinePlus).
Pain when you urinate: it could mean a urinary tract infection (UPMC).
Unusual discharge: green or yellow, or with a bad odor (UPMC).
Fainting spells, dizziness or blurred vision: (Cleveland Clinic).

The CDC’s Hear Her campaign adds urgent warning signs for any stage of pregnancy: a headache that won’t go away or gets worse over time, trouble breathing, chest pain or a fast-beating heart, severe belly pain that doesn’t go away, and thoughts about harming yourself or your baby. Seek medical care immediately.

If something feels wrong and it is not on a list, call anyway. This article is general education, not medical advice.

Frequently Asked Questions

How many weeks is the first trimester?

About 12 to 14 weeks. The Office on Women’s Health uses weeks 1 through 12, Mayo Clinic says about the first 13 weeks, UPMC reports that ACOG ends it at 13 weeks and 6 days, and MedlinePlus says it continues through week 14.

When does the first trimester end?

By the ACOG definition, as reported by UPMC, it ends once you have passed 13 weeks and 6 days, and the second trimester begins at 14 weeks 0 days. Other sources end it after week 12 or week 13, or through week 14.

What are the most common symptoms in early pregnancy?

The Office on Women’s Health lists extreme tiredness, tender and swollen breasts, an upset stomach with or without vomiting, food cravings or aversions, mood swings, constipation, frequent urination, headache and heartburn. It also notes that some women feel no discomfort at all.

What should I avoid in the first three months of pregnancy?

Alcohol, smoking and recreational drugs come first. The Office on Women’s Health also lists deli meats unless steaming hot, unpasteurized milk and soft cheeses, raw sprouts and high-mercury fish. March of Dimes says to limit caffeine to 200 milligrams each day.

When does morning sickness get better?

UPMC says nausea is usually at its worst around week 9, then gets better and often goes away in the second trimester. Eating smaller meals more often can help. Call your provider if vomiting is severe, because too much vomiting may make it hard to keep down fluids.

When is the first prenatal visit?

Call to schedule as soon as you have a positive test. UPMC says the first prenatal visit usually happens around 8 weeks and takes about 45 minutes. Expect a health history, a pelvic exam, and blood and urine tests. An ultrasound follows during the first trimester.

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