Is week 13 the end of the first trimester or the start of the second?

Most doctors consider the end of week 13 to be the close of the first trimester, with the second trimester beginning at week 14. Some sources round to 12 weeks for simplicity, but clinically week 13 is the final week of the first trimester.

What happens in the first trimester?

The first trimester (weeks 1–13) is when the fertilized egg implants, the embryo forms, and major organs begin developing. Common symptoms include morning sickness, fatigue, breast tenderness, and frequent urination. By week 6–8, a heartbeat may be visible on ultrasound. By week 12, the risk of miscarriage drops significantly.

What happens in the second trimester?

The second trimester (weeks 14–27) is often called the 'honeymoon phase' of pregnancy. Morning sickness usually fades, energy returns, and you may feel the baby's first movements (quickening) around week 18–22. The anatomy scan typically happens at week 20. This is when the baby grows rapidly and you begin to show more.

What happens in the third trimester?

The third trimester (weeks 28–40+) is the final stretch. The baby gains most of its weight, lungs mature, and you may experience Braxton Hicks contractions, back pain, and difficulty sleeping. By week 37, the baby is considered full term. Most checkups become weekly or biweekly as the due date approaches.

Can trimesters vary between pregnancies?

The week boundaries of each trimester are standard, but how you experience them can vary greatly between pregnancies and between women. Some have severe first-trimester nausea; others feel fine. Some carry to 41 weeks; others deliver at 37. The trimester framework is a guide, not a rule.

Why is pregnancy tracked by trimesters instead of months?

Trimesters divide pregnancy into three logical stages of fetal development: formation (first), growth (second), and maturation (third). Months are less precise because they vary in length (28–31 days) and because pregnancy is actually about 9.5 calendar months, not exactly 9. Trimesters give doctors and patients a clearer clinical framework.

Pregnancy Trimester Tracker: What Trimester Am I In?

Pregnancy Trimester Tracker

Find out what trimester you are in, how many weeks pregnant you are, and what to expect in each stage of pregnancy.

Educational use only — not medical advice

This tracker provides estimates based on standard pregnancy dating. Always consult your healthcare provider for personalized prenatal care and confirmation of gestational age.

What trimester am I in?

First day of last period (LMP) *

The first day of full bleeding — not spotting.

Average cycle length (days)

Standard is 28 days. Adjust if yours differs.

Pregnancy Trimester Breakdown

First Trimester

Weeks 1–13

Conception, implantation, and the critical early development of all major organs.

Key milestones

Week 4–5: Positive pregnancy test possible
Week 6–8: First ultrasound and heartbeat
Week 9–10: Baby's facial features form
Week 12: End of first trimester, risk of miscarriage drops

Common symptoms

Morning sicknessFatigueBreast tendernessFrequent urinationFood aversions

The neural tube closes, the heart begins beating, and limbs start forming. By week 12, the baby is about the size of a plum.

Second Trimester

Weeks 14–27

Rapid growth, decreased nausea, and the first flutters of baby movement.

Key milestones

Week 14–16: Energy returns, nausea fades
Week 18–22: First baby movements felt (quickening)
Week 20: Anatomy scan at the halfway point
Week 24: Viability milestone — baby can survive outside the womb with care

Common symptoms

Growing bellyBackacheStretch marksNasal congestionIncreased appetite

Hair, eyebrows, and eyelashes grow. The baby can hear your voice. By week 27, the baby is about the size of a rutabaga and weighs roughly 2 pounds.

Third Trimester

Weeks 28–40+

Final maturation, rapid weight gain, and preparation for labor and delivery.

Key milestones

Week 28: Third trimester officially begins
Week 32: Baby practices breathing movements
Week 37: Full term — baby is ready for delivery
Week 40: Estimated due date

Common symptoms

Braxton Hicks contractionsShortness of breathSwellingTrouble sleepingFrequent urination returns

The brain develops rapidly, the lungs mature, and the baby settles into head-down position. By week 40, the baby is about the size of a small pumpkin and averages 7–8 pounds.

Pregnancy Trimester Tracker

A pregnancy trimester tracker helps you understand exactly which stage of pregnancy you are in and what changes are happening in your body and with your baby. Pregnancy is divided into three trimesters, each with unique developmental milestones, symptoms, and care needs. Whether you are trying to confirm what trimester you are in or planning ahead, this tracker gives you a clear week-by-week picture.

How many weeks are in each trimester?

Pregnancy is measured from the first day of your last menstrual period (LMP) and spans approximately 40 weeks. These 40 weeks are grouped into three trimesters:

First Trimester — Weeks 1 to 13

This is approximately 3 months. It begins on the first day of your last period and continues through the end of week 13. During this time, the fertilized egg implants, the embryo forms, and all major organs begin to develop. By the end of the first trimester, the baby is roughly the size of a plum and the risk of miscarriage drops significantly.

Second Trimester — Weeks 14 to 27

This is approximately 3.5 months. The second trimester begins at week 14 and runs through the end of week 27. It is often called the "honeymoon phase" because morning sickness usually fades, energy levels rise, and the baby bump becomes more visible. You may feel the baby's first movements (quickening) between weeks 18 and 22.

Third Trimester — Weeks 28 to 40+

This is approximately 3 months, continuing until delivery. The third trimester starts at week 28 and lasts until birth. The baby gains most of its weight during this period, the lungs mature, and the brain develops rapidly. By week 37, the baby is considered full term. Most prenatal visits become weekly or biweekly as the due date approaches.

What trimester am I in?

To find out what trimester you are in, count the weeks from the first day of your last menstrual period (LMP). Use the calculator above to do this automatically, or follow this simple guide:

First trimester: You are 1 to 13 weeks pregnant.
Second trimester: You are 14 to 27 weeks pregnant.
Third trimester: You are 28 to 40+ weeks pregnant.

If you do not know your LMP, you can also use your estimated due date (EDD) to work backwards. Subtract your remaining days from 280 (40 weeks) to find how many weeks pregnant you are, then match that number to the trimester ranges above.

First Trimester: What to expect

The first trimester is a time of rapid and invisible change. Although you may not look pregnant yet, your body is working intensely. The fertilized egg implants in the uterine wall, the placenta begins forming, and the neural tube — which becomes the brain and spinal cord — closes by week 6.

Common first-trimester symptoms include morning sickness (which can happen at any time of day), extreme fatigue, breast tenderness, mood swings, and frequent urination. Some women also experience food aversions, heightened sense of smell, and mild cramping. By week 8, a transvaginal ultrasound can usually detect a heartbeat. By week 12, the baby has formed all major organs and the risk of miscarriage drops substantially.

Self-care tips: rest as much as possible, eat small frequent meals to manage nausea, stay hydrated, begin taking prenatal vitamins with folic acid, and avoid alcohol, smoking, and unpasteurized foods.

Second Trimester: What to expect

For many women, the second trimester is the most enjoyable phase of pregnancy. Morning sickness typically fades by week 14, energy returns, and the emotional rollercoaster often stabilizes. You will begin to show more obviously, and between weeks 18 and 22, most women feel the baby's first movements — described as flutters, bubbles, or gentle taps.

The anatomy scan, usually performed at week 20, checks the baby's organs, limbs, spine, and growth. You may also learn the baby's sex if you choose to. The baby grows from about the size of a lemon at week 14 to roughly the size of a rutabaga at week 27, and begins to hear sounds from the outside world.

New symptoms may include backache, round ligament pain, nasal congestion, stretch marks, and an increased appetite. This is a great time to start pregnancy-safe exercise, set up your nursery, and discuss birth plans with your provider.

Third Trimester: What to expect

The third trimester is the home stretch. The baby gains approximately half a pound per week, the lungs mature in preparation for breathing, and the brain develops rapidly. The baby will likely settle into a head-down position by week 32–36.

Physically, this trimester can be the most challenging. Braxton Hicks contractions — practice contractions — become more frequent. You may experience shortness of breath as the uterus presses against your diaphragm, swelling in your feet and ankles, heartburn, trouble sleeping, and frequent urination as the baby's head drops into the pelvis.

By week 37, the baby is considered full term, meaning the lungs and other organs are developed enough for life outside the womb. Prenatal visits become weekly, and your provider will monitor for signs of labor, check the baby's position, and discuss delivery options.