Pregnancy Trimester Guide: What Happens Each Stage

Updated March 2026 · By the FertilityCalcs Team

Pregnancy is divided into three trimesters, each lasting about 13 weeks. Every trimester brings distinct changes in your body, new stages of baby development, and different medical tests and milestones. Whether you are newly pregnant or approaching your due date, understanding what to expect during each phase helps you feel prepared, know when something needs attention, and make the most of every prenatal visit.

First Trimester: Weeks 1-13

The first trimester is when the most critical development occurs. By week 5, the embryo heart begins to beat. By week 8, all major organs have started forming. By the end of the first trimester at week 13, the baby is about 3 inches long, has fingers and toes, and can make small movements. This is also when the risk of miscarriage is highest, dropping significantly after a heartbeat is confirmed at 8 to 10 weeks.

Common symptoms include nausea and morning sickness (which can occur at any time of day), extreme fatigue, breast tenderness, frequent urination, and heightened sense of smell. Many women also experience food aversions and emotional swings due to rapidly changing hormone levels. These symptoms typically peak around weeks 8 to 10 and begin to ease by week 13.

Weeks 5-6: heartbeat detectable on ultrasound
Weeks 8-10: all major organs forming
Week 10-13: NIPT blood test for chromosomal screening
Week 11-14: nuchal translucency ultrasound
Common symptoms: nausea, fatigue, breast tenderness

Pro tip: Keep small, frequent meals to manage nausea. Protein-rich snacks, ginger, and vitamin B6 are evidence-backed remedies. Talk to your provider if nausea is severe or you cannot keep fluids down.

Second Trimester: Weeks 14-27

The second trimester is often called the easiest phase. Morning sickness typically subsides, energy returns, and you begin to feel the baby move between weeks 18 and 22. The baby grows from about 3.5 inches to 14 inches and develops features like eyebrows, eyelashes, fingerprints, and the ability to hear sounds.

This is when you will have your anatomy scan ultrasound around week 20, which checks all major organs and can reveal the sex of the baby if you want to know. The glucose tolerance test for gestational diabetes is typically done between weeks 24 and 28. Your belly becomes visibly pregnant during this trimester, and you may start experiencing round ligament pain, back pain, and skin changes.

Weeks 18-22: first fetal movements felt
Week 20: anatomy scan ultrasound
Week 24-28: glucose tolerance test for gestational diabetes
Baby can hear sounds by week 22-23
Common symptoms: round ligament pain, back pain, visible bump

Pro tip: The anatomy scan is one of the most detailed ultrasounds of your pregnancy. Bring a list of questions for your sonographer and do not hesitate to ask for explanations of what you are seeing on screen.

Third Trimester: Weeks 28-40

The third trimester is a time of rapid growth and final preparation. The baby gains most of its weight during these weeks, growing from about 2 pounds at week 28 to 6-9 pounds at birth. The lungs mature, the brain develops rapidly, and the baby begins to settle into a head-down position for delivery, usually by weeks 32 to 36.

You will have more frequent prenatal visits during the third trimester, moving to every two weeks and then weekly as you approach your due date. The Group B strep test is done between weeks 35 and 37. Common symptoms include shortness of breath, heartburn, swelling in the feet and ankles, Braxton Hicks contractions, and difficulty sleeping.

Week 28: Rh antibody screen, start kick counts
Week 32-36: baby moves to head-down position
Week 35-37: Group B strep test
Week 37: considered early term
Week 39-40: full term, delivery expected

Pro tip: Start doing daily kick counts from week 28. You should feel at least 10 movements in a 2-hour window. If you notice a significant decrease in movement, contact your provider immediately.

Key Prenatal Tests and Screenings

Prenatal testing provides important information about your health and the baby development. First trimester screening includes blood tests and the nuchal translucency ultrasound to assess risk for chromosomal conditions like Down syndrome. NIPT (non-invasive prenatal testing) is a blood test that can detect chromosomal abnormalities with high accuracy as early as week 10.

Second trimester tests include the quad screen blood test and the detailed anatomy scan ultrasound. Third trimester testing includes the glucose tolerance test, Group B strep culture, and more frequent monitoring of baby position and growth. If any screening test indicates elevated risk, diagnostic tests like amniocentesis or CVS can provide definitive answers.

Week 10+: NIPT blood test (optional, high accuracy)
Week 11-14: nuchal translucency ultrasound
Week 15-20: quad screen blood test
Week 20: anatomy scan ultrasound
Week 24-28: glucose tolerance test
Week 35-37: Group B strep test

Pro tip: Ask your provider which tests are routine at your practice and which are optional. Understanding the difference between screening tests (which estimate risk) and diagnostic tests (which provide definitive answers) helps you make informed decisions.

When to Call Your Provider

While most pregnancy symptoms are normal, certain signs warrant immediate medical attention. In the first trimester, contact your provider for heavy bleeding, severe abdominal pain, or persistent vomiting that prevents keeping any food or fluids down. In the second trimester, watch for fluid leaking from the vagina, regular contractions before week 37, or a sudden severe headache with vision changes.

In the third trimester, decreased fetal movement, regular contractions that increase in frequency, fluid leaking, vaginal bleeding, severe headache with swelling, and upper abdominal pain all require immediate attention. Trust your instincts. Providers would rather evaluate a false alarm than miss a genuine complication.

Any trimester: heavy bleeding, severe pain, high fever
Second trimester: fluid leaking, contractions before 37 weeks
Third trimester: decreased movement, regular contractions, severe headache with swelling
Always call: sudden vision changes, difficulty breathing, chest pain

Pro tip: Save your OB office number, the after-hours nurse line, and your hospital labor and delivery number in your phone. Having these readily available reduces stress when you need guidance.

Frequently Asked Questions

When does each trimester start and end?

The first trimester spans weeks 1-13, the second trimester weeks 14-27, and the third trimester weeks 28-40. Pregnancy is dated from the first day of your last menstrual period, so the first two weeks are actually before conception occurs.

Which trimester is the hardest?

Most women find the first trimester hardest due to nausea and fatigue, or the third trimester hardest due to physical discomfort and sleep difficulties. The second trimester is generally considered the most comfortable, earning it the nickname the "honeymoon trimester."

When will I start showing?

First-time mothers typically start showing between weeks 12 and 16. Second or subsequent pregnancies often show earlier, sometimes by week 8-10, because the abdominal muscles have been stretched before. Body type, weight, and how the baby is positioned also affect timing.

Is it normal to have cramps during pregnancy?

Mild cramping is common, especially in the first trimester as the uterus grows and in the second trimester from round ligament stretching. However, severe or persistent cramping, especially with bleeding, should be reported to your provider promptly.

How often are prenatal appointments?

The typical schedule is monthly visits through week 28, every two weeks from weeks 28-36, and weekly from week 36 until delivery. High-risk pregnancies may require more frequent monitoring. Each visit checks blood pressure, weight, urine, and fetal heart tones.