Pregnancy week by week: the complete guide to all 40 weeks

The most important number in pregnancy

Finding out you're pregnant triggers an instant mix of excitement and overwhelm. Every pregnant person becomes intimately familiar with one number: their current week. When is my due date? What's happening inside my body? Is this symptom normal?

A healthy pregnancy lasts roughly 40 weeks, and every single week brings precise biological changes. You're not actually pregnant for nine months. You're pregnant for forty weeks, which is closer to ten. And the counting starts two weeks before you conceive.

This guide breaks down all 40 weeks of pregnancy across the three trimesters, what to expect at each stage, and how to use the August AI Pregnancy Calculator to find your exact week.

Not sure how many weeks pregnant you are? Use the August AI Pregnancy Calculator to find your week and estimated due date in under 30 seconds.

How pregnancy weeks are counted

Pregnancy is calculated from the first day of your last menstrual period (LMP), not from the day you conceived. This is the standard medical convention used worldwide.

Why this can feel strange: you typically conceive about 2 weeks after the start of your last period. So when your doctor says you're "6 weeks pregnant," conception happened about 4 weeks ago. Your baby is biologically 4 weeks old, but your pregnancy is dated as 6 weeks.

Why doctors count this way: most people can remember the first day of their last period accurately. Conception date is harder to pinpoint, even with ovulation tracking.

When the dating gets adjusted:

• Ultrasound dating. First-trimester ultrasounds adjust your due date when LMP-based dating is off by more than 5 to 7 days.
• IVF pregnancies. Conception date is known exactly, so dating uses the embryo transfer date plus standard development timeline.
• Irregular cycles. People with irregular cycles often need ultrasound dating because LMP-based calculations can be off.

For deeper detail on due date calculation, see What to Expect and The Bump.

The three trimesters: a quick map

A standard pregnancy divides into three trimesters of roughly 13 weeks each.

First trimester (weeks 1 to 12). Conception, implantation, and early organ formation. This trimester carries the most intense hormonal symptoms and the highest miscarriage risk.

Second trimester (weeks 13 to 26). Often called the honeymoon phase. Morning sickness eases, energy returns, and the baby grows rapidly.

Third trimester (weeks 27 to 40+). The home stretch. Your baby focuses on weight gain and lung maturation while physical symptoms return.

Each trimester has its own rhythm. The sections below cover what to expect in each.

First trimester: weeks 1 to 12

The first trimester is a whirlwind of biological activity. Across weeks 1 through 12, your baby transitions from dividing cells into a fully structured fetus.

What's happening biologically:

• Weeks 1 to 4. Your body prepares for ovulation, conception occurs, and the blastocyst implants in the uterine wall by week 4.
• Weeks 5 to 8. All major organ systems begin forming. A primitive heart tube starts beating around week 6.
• Weeks 9 to 12. The embryo becomes a fully formed fetus. Fingers, toes, and facial features become visible on ultrasound.

Common first trimester symptoms, driven by surging human chorionic gonadotropin (hCG), the pregnancy hormone:

• Profound fatigue
• Nausea and vomiting (peaks weeks 6 to 10)
• Sore, swollen breasts
• Frequent urination
• Heightened sense of smell and food aversions
• Mood changes

Key appointments and milestones:

• First prenatal visit (weeks 8 to 10): blood work and first ultrasound
• First-trimester screenings (weeks 11 to 13) for chromosomal conditions
• Week 12 marks a sharp decline in miscarriage risk

The highest miscarriage risk is in these early weeks, and most losses happen for reasons outside your control. If you're feeling anxious about it, our guide on understanding miscarriage risk may help. For the full week-by-week first trimester guide, see The Bump and What to Expect.

Second trimester: weeks 13 to 26

The second trimester is often called "the honeymoon trimester." Energy returns, nausea eases, and the baby grows rapidly.

What's happening biologically:

• Weeks 13 to 16. The fetus rapidly grows. Biological sex can be identified on ultrasound.
• Weeks 17 to 20. First fetal movements ("quickening") begin, typically between weeks 18 and 22 for first-time moms. The anatomy scan around week 20 evaluates structural growth.
• Weeks 21 to 26. Bone development accelerates and a sleep-wake cycle establishes. By week 24, the fetus reaches the threshold of viability.

Common changes:

• Energy typically returns
• Morning sickness usually eases
• Visible baby bump emerges
• Round ligament pain as the uterus grows
• Possible heartburn and indigestion

Key appointments and milestones:

• Anatomy ultrasound (weeks 18 to 22)
• Glucose challenge test (weeks 24 to 28) for gestational diabetes
• Viability threshold (week 24)
• Quickening (weeks 18 to 22 for first pregnancies)

For the second trimester guide, see The Bump and Happiest Baby.

Third trimester: weeks 27 to 40

The third trimester is the home stretch. Most prenatal appointments cluster here.

What's happening biologically:

• Weeks 27 to 32. The fetus gains weight rapidly. Lungs continue maturing. Brain development accelerates.
• Weeks 33 to 36. Uterine space becomes limited. The baby maneuvers into a head-down position to prepare for labor.
• Weeks 37 to 40. Week 37 marks full term pregnancy. Your baby is ready for birth at any point ahead of week 40.

Common third trimester symptoms:

• Return of fatigue
• Back pain and pelvic discomfort
• Swelling in feet and hands
• Braxton Hicks ("practice") contractions
• Difficulty sleeping
• Heartburn often worsens

Key appointments and milestones:

• Increasing appointment frequency (every 2 weeks, then weekly after week 36)
• Group B strep test (around week 36)
• Position checks (head-down by week 36)
• Term pregnancy begins (week 37)
• Estimated due date (week 40)

For the full week-by-week third trimester guide, see The Bump and Tiny Hearts.

Beyond 40 weeks: what happens if you go past your due date

Your due date is an estimate, not a deadline. Only about 5% of babies arrive on their predicted due date. About 60% are born between weeks 39 and 41. Reaching 41 weeks pregnant without active labor is normal.

If you go past 40 weeks:

• Week 41. Most providers begin discussing induction options. Increased monitoring (non-stress tests, biophysical profiles) typically begins.
• Week 42. Most providers will not allow pregnancy past week 42 due to rising risks.

A pregnancy is considered "post-term" once it passes 42 weeks. With modern monitoring, this is rare.

For deeper guidance on the past-due-date stage, see Happiest Baby and What to Expect.

How to use the August AI Pregnancy Calculator

The August AI Pregnancy Calculator answers the questions every pregnant person asks throughout pregnancy:

• How many weeks pregnant am I right now?
• What's my estimated due date?
• What trimester am I in?
• What week will I be on a specific future date?

It functions as both a pregnancy calculator and a due date calculator, working with multiple input methods:

• Last menstrual period (LMP): the standard method most providers use
• Conception date: if you know when you conceived
• Ultrasound dating: if your provider has adjusted your due date
• IVF transfer date: for IVF pregnancies, with embryo age accounted for

Once calculated, the tool shows your current week, your trimester, your estimated due date, and what pregnancy milestones are coming up next.

Try the August AI Pregnancy Calculator.

One last thing

Forty weeks feels like forever when you're at week 6 and a blink when you're at week 38. The August AI Pregnancy Calculator is built to make every week of that journey easier to track. Bookmark it. You'll use it more than you expect.