Pregnancy by Trimester: A Week-by-Week Guide

Pregnancy is divided into three trimesters across about 40 weeks. The first trimester runs from weeks 1 to 13, a time of early development that often brings fatigue and nausea. The second, weeks 14 to 27, is when many people feel their best and first notice the baby move. The third, weeks 28 to 40, is a stretch of rapid growth as the body prepares for birth. Every pregnancy is a little different, so treat these as general milestones rather than a fixed schedule.

Pregnancy by Trimester: The Three Stages

The simplest way to picture pregnancy is as three phases, called trimesters, spread across about 40 weeks counted from the first day of your last menstrual period. The first trimester covers weeks 1 through 13, the second covers weeks 14 through 27, and the third covers weeks 28 through 40. Each phase has its own general pattern of growth for the baby and its own common changes for the pregnant person.

These week ranges are the standard way clinicians describe a pregnancy, and they line up with the milestones you will read about below. If you want to know exactly which week and trimester you are in right now, the Pregnancy Calculator can place you on the timeline from a single date. Remember that the boundaries are approximate, and healthy pregnancies still vary in how they unfold.

The table below gives a side-by-side overview of all three trimesters so you can see the whole journey at a glance before we walk through each stage in more detail.

First Trimester: Weeks 1 to 13

The first trimester is a period of quiet but remarkable change. From a single fertilized cell, the embryo develops the beginnings of the brain, spine, heart, and other major organs. By the end of this stage the baby is roughly the size of a small plum and already has a steady heartbeat. Much of the foundational work of pregnancy happens now, even though there is often little to see from the outside.

For the pregnant person, the first trimester is when many of the well-known early symptoms appear. Fatigue can feel heavy and sudden, and nausea, sometimes called morning sickness even though it can occur at any hour, is common. Tender breasts, a heightened sense of smell, and frequent trips to the bathroom are also typical. These symptoms are usually a normal part of the body adjusting, and they often ease as the weeks pass.

This is also the stage when prenatal care begins. An early visit confirms the pregnancy, estimates a due date, and sets up the schedule of checkups ahead. If you are curious about how that date is worked out, our sibling guide explains how a due date is calculated from your last period or an early ultrasound.

Second Trimester: Weeks 14 to 27

Many people describe the second trimester as the most comfortable stretch of pregnancy. Nausea and extreme tiredness often fade, energy tends to return, and the pregnancy starts to feel more visible and real. The belly grows in a noticeable way during these weeks, and everyday routines usually become easier to manage than they were in the early months.

The baby is growing quickly and becoming more active. Facial features sharpen, and the baby can move, swallow, and begin to hear sounds. Somewhere in the middle of this trimester, often between weeks 18 and 22 for a first pregnancy, many people feel the first flutters of movement, a milestone sometimes called quickening. A mid-pregnancy ultrasound during this window checks growth and development and can often reveal the baby’s sex if you wish to know.

Steady, healthy weight gain is expected in the second trimester as the baby and supporting tissues grow. Providers track this at each visit because it is one useful sign that things are on track. To understand the general pattern and what tends to be typical, see our sibling overview of pregnancy weight gain by stage.

Third Trimester: Weeks 28 to 40

The third trimester is the home stretch, marked by rapid growth as the baby puts on most of its birth weight. The lungs and brain continue to mature, and in the final weeks the baby usually settles into a head-down position ready for delivery. This is the longest run of steady physical change for the baby, packed into the closing months.

As the baby grows larger, the pregnant person often feels the effects more. Back pain, shortness of breath, swelling in the feet, heartburn, and trouble finding a comfortable sleeping position are all common in these weeks. Prenatal visits become more frequent so that the provider can monitor the baby’s position, growth, and heartbeat, and discuss a plan for labor and delivery.

Toward the very end, the body offers early signs that labor may be approaching, such as the baby dropping lower or occasional practice contractions. A full-term pregnancy can arrive any time from a few weeks before the due date to a week or two after it, and your provider will guide you on what to watch for and when to reach out.

Prenatal Care Across All Three Trimesters

Regular prenatal care is the thread that runs through every trimester. These checkups let a provider follow the baby’s growth, track your health, offer routine tests and screenings, and answer the questions that come up along the way. Early and consistent care is one of the most helpful things you can do during pregnancy.

The schedule usually starts with monthly visits, moves to every two weeks in the later part of pregnancy, and becomes weekly near the end. Between visits, providers generally encourage a balanced diet, recommended prenatal vitamins, gentle activity when appropriate, and plenty of rest. Your own care plan may look different, and your provider will tailor it to you.

Above all, reach out sooner rather than later if something worries you. You do not need to wait for the next scheduled appointment to ask about symptoms that feel new, severe, or simply unsettling. Trusting that instinct is part of good care.

FAQs About Pregnancy Trimesters

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Educational note: This article is general information for learning about pregnancy stages, not medical advice, and it is not a substitute for care from a qualified professional. Every pregnancy is different, and the week ranges and milestones described here are general patterns that will not fit everyone exactly. Please attend your prenatal appointments and contact your healthcare provider with any question or concern about your pregnancy or your health. Reviewed for general accuracy by Dr. Abdullah Khalil, MBBS, as part of our editorial review process. Content last reviewed September 10, 2026.