The first trimester of pregnancy lasts 13 weeks and 6 days, ending at 14 weeks. It begins on the first day of your last menstrual period and continues through week 13. This is the first of three trimesters, each lasting roughly three months, that make up a full 40-week pregnancy.
How Long Is The 1st Trimester Of Pregnancy
Pregnancy is measured from the first day of your last menstrual period, not from the date of conception. This means you are already considered about 2 weeks pregnant when conception actually occurs. The first trimester spans weeks 1 through 13 — a total of 13 weeks and 6 days.
Doctors use this dating method because the exact date of conception is rarely known. The last menstrual period provides a consistent starting point. Most pregnancies last 40 weeks when counted this way, though only about 5% of babies arrive on their exact due date.
What Happens During the First Trimester
The first trimester is the most active period of fetal development. In just 13 weeks, a single fertilized egg develops into a fetus with functioning organs, a beating heart, and recognizable human features.
By week 4, the neural tube — which becomes the brain and spinal cord — begins forming. The heart starts beating around week 6. By week 8, all major organs are beginning to develop. Arms, legs, fingers, and toes appear between weeks 8 and 10. By the end of week 12, the fetus has all essential organs and body systems in place, though they continue to mature throughout the rest of the pregnancy.
At the end of the first trimester, the fetus measures about 3 inches long and weighs roughly 1 ounce. It can make facial expressions and may suck its thumb, though you cannot feel these movements yet.
Common First Trimester Symptoms
Hormone levels rise rapidly during the first trimester, which drives most early pregnancy symptoms. These symptoms vary widely. Some women experience many; others experience almost none.
Common symptoms include:
- Nausea and vomiting — often called morning sickness, though it can occur at any time of day
- Breast tenderness and swelling
- Fatigue and exhaustion
- Frequent urination
- Food aversions and cravings
- Mood swings
- Constipation
- Heightened sense of smell
Morning sickness typically peaks around weeks 8-10 and improves by the end of the first trimester for most women. The exact cause is not fully understood, but rising levels of the hormone hCG and estrogen are thought to play a role.
Severe nausea and vomiting that prevents eating or drinking is a condition called hyperemesis gravidarum. This affects about 1-3% of pregnancies and requires medical attention. If you cannot keep fluids down or are losing weight, contact your healthcare provider.
First Trimester Prenatal Appointments and Tests
Your first prenatal visit usually happens between weeks 8 and 10. This appointment establishes your due date, confirms the pregnancy, and screens for any health conditions that could affect you or the baby.
The first visit typically includes a physical exam, blood tests, urine tests, and an ultrasound. The ultrasound confirms the gestational age and checks for a fetal heartbeat. Blood tests check blood type, Rh factor, hemoglobin levels, and screen for infections such as HIV, hepatitis B, and syphilis.
Screening tests offered during the first trimester include:
- First-trimester combined screening — a blood test and nuchal translucency ultrasound performed between weeks 11 and 13
- Cell-free DNA testing — a blood test that screens for chromosomal conditions like Down syndrome, available after week 10
- Chorionic villus sampling — a diagnostic test offered between weeks 10 and 13 for women at higher risk of genetic conditions
These first-trimester screenings are optional. Your provider will explain which tests are recommended based on your age, medical history, and risk factors.
When Is the First Trimester Over?
The first trimester ends at week 14. In pregnancy counting, that means you have completed 13 weeks and 6 days. Your 14th week marks the beginning of the second trimester.
Your healthcare provider will calculate your gestational age at your first ultrasound. If your ultrasound measurements differ from your last menstrual period dates, the ultrasound date is used to determine your official pregnancy timeline.
The transition to the second trimester brings relief for many women. Nausea often improves, energy levels return, and the risk of miscarriage drops significantly. The miscarriage rate is highest in the first trimester, with most losses occurring before week 12.
What to Avoid During the First Trimester
The first trimester is a critical window for fetal development. Certain substances and activities carry known risks during this period.
Alcohol should be completely avoided. No amount of alcohol has been proven safe during pregnancy. Fetal alcohol spectrum disorders can cause lifelong physical and cognitive problems.
Smoking and vaping should also stop. Tobacco use during pregnancy increases the risk of miscarriage, preterm birth, low birth weight, and placental complications.
Certain foods carry foodborne illness risks that are especially dangerous during pregnancy:
- Raw or undercooked meat, poultry, and seafood
- Unpasteurized dairy products and juices
- Deli meats and hot dogs unless heated until steaming
- Raw sprouts
- High-mercury fish such as shark, swordfish, king mackerel, and tilefish
Listeria and toxoplasmosis infections can cross the placenta and harm the developing fetus. Washing produce thoroughly and cooking meat to safe internal temperatures reduces these risks.
Check with your healthcare provider before taking any medication, including over-the-counter drugs and supplements. Some medications that are safe outside pregnancy can cause birth defects during the first trimester.
When to Call Your Doctor
Some symptoms during the first trimester require prompt medical evaluation. Contact your provider if you experience:
- Heavy vaginal bleeding — soaking through a pad in an hour
- Severe abdominal or pelvic pain
- Fever of 100.4°F or higher
- Severe vomiting with inability to keep fluids down
- Pain or burning during urination
- Sudden severe headache or vision changes
Light spotting can be normal in early pregnancy, especially around the time your period would have been due. But any bleeding should be reported to your provider to rule out complications.
Why the First Trimester Feels Long
Many women find the first trimester psychologically challenging. Symptoms are often at their worst, the pregnancy may not be visibly showing, and many women choose not to share the news until the risk of miscarriage decreases.
The combination of feeling unwell, keeping the pregnancy private, and waiting for the second trimester can make these 13 weeks feel longer than they are. The physical discomfort is real, but so is the rapid development happening inside. By week 14, the hardest part of fetal development is complete, and the risk of early pregnancy loss drops substantially.
Frequently Asked Questions
How many weeks is the first trimester?
The first trimester is 13 weeks and 6 days, spanning weeks 1 through 13 of pregnancy. It ends when you enter week 14.
When does the first trimester start?
The first trimester starts on the first day of your last menstrual period, not the date of conception. This means you are about 2 weeks pregnant when conception actually occurs.
When does morning sickness end?
Morning sickness typically improves by the end of the first trimester, around weeks 12-14. Some women continue to experience nausea into the second trimester.
Is the first trimester the highest risk period?
Yes, the first trimester carries the highest risk of miscarriage, with most losses occurring before week 12. The risk drops significantly once the second trimester begins.
