Pregnancy Week by Week: Your Essential 40-Week Guide

Pregnancy is measured in 40 weeks, grouped into three trimesters. The first trimester (weeks 1–12) is when your baby’s major organs form, the second (weeks 13–27) brings your first movements and the anatomy scan, and the third (weeks 28–40) is rapid growth and getting ready for birth. This pregnancy week by week guide walks you through what to expect at every stage.

Following your pregnancy week by week helps you understand what’s normal, know which appointments are coming up, and feel more in control of a process that can otherwise feel like a mystery. Below you’ll find a plain-English overview of all three trimesters, a timeline of the checkups you can expect, and links to detailed week-by-week guides for each stage.

How pregnancy weeks are counted

Here’s the part that confuses almost everyone: pregnancy is dated from the first day of your last menstrual period (LMP), not from the day you conceived. That means “week 1” happens roughly two weeks before egg and sperm actually meet. Providers use this method because most people know when their last period started, but not the exact moment of conception.

The practical takeaways:

• A full-term pregnancy lasts about 40 weeks from your LMP — roughly nine calendar months.
• Your estimated due date falls at the end of week 40.
• “Term” begins at 37 weeks; 39–40 weeks is considered full term.
• Only about 1 in 20 babies actually arrives on the due date — anywhere from 37 to 42 weeks is common, so treat it as a target, not a deadline.

The three trimesters at a glance

As you follow your pregnancy week by week, each trimester has its own character, symptoms, and milestones:

• First trimester — weeks 1 to 12. The foundational stage. Your baby grows from a single cell into a fully formed (if tiny) fetus with a beating heart. This is also when early symptoms like nausea and fatigue tend to peak.
• Second trimester — weeks 13 to 27. Often the most comfortable stretch. Nausea usually eases, energy returns, your bump becomes visible, and you feel those first flutters of movement.
• Third trimester — weeks 28 to 40. The home stretch. Your baby gains weight quickly and their lungs and brain finish maturing while you prepare for labor and birth.

First trimester overview (weeks 1–12)

In the first weeks, your baby is a rapidly dividing cluster of cells that implants in the uterus and begins forming the neural tube — the early structure that becomes the brain and spinal cord. A heartbeat can often be detected around week 6, and by the end of week 8 the major organs and limb buds are in place. By week 12 your baby is around 2–3 inches long, about the size of a lime, with fingers, toes, and recognizable facial features.

For you, this trimester is frequently the most physically demanding. Morning sickness (which can strike at any hour), exhaustion, tender breasts, and frequent urination are all common as pregnancy hormones surge. Your first prenatal visit and dating ultrasound usually happen during this window. For the full breakdown, see our first trimester week by week guide.

Second trimester overview (weeks 13–27)

Many people call the second trimester the “honeymoon” phase. Nausea typically fades, your appetite and energy come back, and the pregnancy starts to feel real in a good way. Somewhere between weeks 16 and 22 you’ll likely feel your baby move for the first time — often described as flutters or bubbles. The detailed anatomy scan, usually around weeks 18–22, checks your baby’s development and is often when you can learn the sex if you choose to.

Your baby grows hair, eyebrows, and eyelashes, begins to hear sounds, and settles into sleep-and-wake cycles. By week 27 they’re roughly the size of a head of cauliflower and weigh around 2 pounds. Read more in our second trimester week by week guide.

Third trimester overview (weeks 28–40)

The final trimester is all about growth and readiness. Your baby puts on weight rapidly, their lungs mature, and they usually settle head-down in preparation for birth around weeks 34–36. You may notice more heartburn, swelling, shortness of breath, and stronger Braxton Hicks (practice) contractions. Prenatal visits become more frequent, moving to every two weeks and then weekly as your due date approaches.

This is the time to pack your hospital bag, finalize your birth preferences, and learn the signs of labor. By week 40 your baby is typically 19–21 inches long and weighs somewhere between 6 and 9 pounds. See our third trimester week by week guide for the full picture.

Your prenatal appointment timeline

Every pregnancy is managed a little differently, but here is roughly how your key appointments fall as your pregnancy progresses week by week:

• Weeks 8–12: First prenatal visit and dating ultrasound.
• Weeks 10–13: Optional early genetic screening (such as NIPT).
• Weeks 18–22: Anatomy scan; sex can often be identified.
• Weeks 24–28: Glucose screening for gestational diabetes.
• Around week 36: Group B strep test.
• Weeks 36–40: Weekly checkups and monitoring until birth.

Always follow the schedule your own provider gives you — they’ll tailor it to your health and any specific needs. For a trusted overview of routine prenatal care, the American College of Obstetricians and Gynecologists (ACOG) is a reliable reference.

Tracking your pregnancy week by week

Forty weeks pass faster than you’d think, and the details blur: the week you first heard the heartbeat, the day you felt that first kick, how your cravings changed month to month. Writing them down as they happen is the only reliable way to hold onto them. A dedicated pregnancy journal that follows your pregnancy week by week gives you a structured place to record each week’s symptoms, appointments, and firsts — and becomes a keepsake you’ll return to for years. Recording your pregnancy week by week means nothing gets lost to memory. If you’re not sure where to start, our guide on what to write in a pregnancy journal is a good next stop.

Frequently asked questions

How many weeks is a full pregnancy?

A full-term pregnancy is about 40 weeks counted from the first day of your last period, which works out to roughly nine months. Babies born from 37 weeks onward are considered term.

Why does pregnancy start before conception?

Pregnancy is dated from your last menstrual period because that date is easier to pinpoint than the moment of conception. As a result, the first two weeks technically come before your baby is conceived.

When will I feel my baby move?

Most people feel their first movements between weeks 16 and 22. If it’s not your first pregnancy, you may notice it earlier because you know what the sensation feels like.

When can I find out my baby’s sex?

The anatomy scan around weeks 18–22 can usually reveal the sex. Some blood-based genetic screening tests can indicate it earlier, from around week 10, if you choose to have them.

Is it normal for symptoms to change each week?

Yes. Symptoms shift constantly week by week as your hormones and your baby’s size change. Nausea and fatigue often ease after the first trimester, while heartburn and swelling tend to appear later. If anything worries you, contact your provider.

Medical disclaimer: This article is for general information only and is not a substitute for professional medical advice. Every pregnancy is different. Always consult your doctor or midwife about your own care, and seek medical attention for any symptoms that concern you.

About the author: Created by a certified prenatal wellness coach and mother of two, in collaboration with OB-GYN consultants and experienced midwives. Content medically reviewed for accuracy, drawing on guidance from established sources including ACOG, the NHS, and the Mayo Clinic.