How Are The Trimesters Of Pregnancy Divided? | Trimester Map

People talk about “first trimester” and “third trimester” like everyone agrees on the exact lines. Most of the time, they do. Still, you’ll see slightly different week ranges depending on the clinic, the book, or the app. That can feel messy when you’re trying to track symptoms, book scans, or make sense of test results.

This article clears it up in plain language. You’ll learn the standard way trimesters are split, why the clock starts where it starts, and why a few sources shift the cutoffs by a week. You’ll also get a quick way to translate “weeks pregnant” into where you are in the trimester timeline.

How pregnancy weeks are counted

Most pregnancy timelines use gestational age. That means the count starts on day one of your last menstrual period, not on the day fertilization happened. Many people ovulate around two weeks after that first day, so “week 2” often lands before conception in a typical cycle. It sounds odd at first, but it gives care teams a shared yardstick they can use for everyone.

If you conceived through IVF, your clinic may talk about an “embryo age” tied to transfer day. Even then, your records usually convert back to gestational age for scans, lab reference ranges, and due date estimates.

How Are The Trimesters Of Pregnancy Divided? By weeks and milestones

The most common breakdown is:

• First trimester: week 1 through week 12
• Second trimester: week 13 through week 27
• Third trimester: week 28 through birth

This is the split you’ll see on many U.S. federal health pages and major medical references. The HHS Office on Women’s Health stages of pregnancy page groups the weeks into these three parts, and the NICHD pregnancy factsheet describes trimesters as week blocks across a 40-week pregnancy.

Some books and clinics label the first trimester as weeks 1–13, the second as 14–27, and the third as 28–40. That doesn’t change what’s happening in the body. It’s a counting choice, often tied to the idea that each trimester is “about 13 weeks.” If you ever see a mismatch, check whether the source is using completed weeks (what you’ve finished) or current week (what you’re in right now).

How pregnancy trimesters are divided in real calendars

Once you know the week ranges, the next question is how they fit into everyday life. The labels are calendar-based, so a trimester can start mid-week. If you track by a weekly app update, your “new trimester” moment may land on a Tuesday, not on a neat month change.

A good rule is to follow your clinical week number. If your chart says 13w0d, you’re at the start of the second trimester under the 13–27 split. If it says 12w6d, you’re still in the first. This small detail matters more than a month label when you’re lining up time-sensitive tests.

What each trimester line is trying to capture

Trimester labels are a shorthand for changes that tend to cluster in time: early development, a middle stretch where many people feel steadier, and the final stretch when growth and birth prep take center stage. The boundaries are not magic doors. They are calendar markers that help people talk about care schedules and typical changes.

First trimester is the setup phase

The first trimester runs from the start of the last period through week 12. In this window, implantation happens, the placenta starts forming, and the embryo’s core structures begin taking shape. Many early symptoms also concentrate here: fatigue, nausea, breast tenderness, mood swings, and smell sensitivity.

You may not have your first prenatal appointment until several weeks in. That gap can feel long. Still, the early weeks are packed with behind-the-scenes change. A plain-English overview of early fetal growth by week appears in MedlinePlus fetal development, and ACOG also outlines major milestones across pregnancy on its patient page about growth and development.

Second trimester is the building phase

The second trimester spans weeks 13–27. For many people, nausea eases and energy ticks up. The uterus rises out of the pelvis, a bump becomes easier to spot, and fetal movement often becomes noticeable in the mid-teens to early-20s weeks. Screening and anatomy scans also commonly land here, so this trimester can feel appointment-heavy even when day-to-day symptoms are calmer.

Third trimester is the finishing phase

The third trimester starts at week 28 and runs until birth. Growth speeds up, the baby’s lungs and brain mature, and your body shifts toward labor readiness. Sleep can get trickier, heartburn can show up, and swelling may appear in hands or feet. Toward the end, visits often become more frequent as the due date nears.

Why some sources shift the trimester cutoffs

If you’ve seen charts that call week 13 “second trimester” while others keep week 13 in the first, you’re not alone. Three things cause most of the confusion:

1. Completed vs current week. “12 weeks pregnant” can mean you completed 12 full weeks and are entering week 13, or it can mean you are somewhere inside week 12.
2. Rounding for 13-week blocks. Forty weeks doesn’t split into three perfectly even chunks. Some charts aim for 13-13-14 week slices, which nudges the labels.
3. Due date dating method. If your due date is set by ultrasound rather than last period, your week count can shift. The trimester labels follow the weeks, not the method.

The practical fix is simple: when a clinic says “second trimester screening” or “third trimester labs,” ask which gestational weeks they use for that test. Labs and scans run on week windows, so the week number matters more than the trimester label.

Trimester timeline by weeks and common checkpoints

This table uses the widely used week split (1–12, 13–27, 28–birth) and pairs it with checkpoints you may hear about. It’s meant as a map, not a care plan. Your clinician may schedule things differently based on your history and the standard practices where you live.

Weeks (gestational age)	Where you are	Common checkpoints you may hear
1–4	First trimester	Dating starts from last period; early test turns positive near end of this range
5–8	First trimester	Early ultrasound may confirm location and heartbeat timing varies
9–12	First trimester	First prenatal visit often lands here; early screening choices may be reviewed
13–16	Second trimester	Nausea often eases; bump may start showing; some blood tests fall here
17–20	Second trimester	Anatomy scan commonly scheduled around this window
21–27	Second trimester	Movement feels clearer; glucose screening is often scheduled in the mid-20s weeks
28–32	Third trimester	Third-trimester labs may start; growth checks may be planned as needed
33–36	Third trimester	Position checks; birth planning talks often pick up
37–41+	Third trimester	Visits often get closer together; plans for labor timing may be reviewed

How trimesters relate to months

Months are a rough translation. Calendar months vary in length, and pregnancy weeks do not line up cleanly with month boundaries. That’s why clinicians talk in weeks. If someone says “I’m five months,” they may mean anything from the late teens to the early 20s weeks.

A simple way to keep your bearings is to use weeks as the base and months as a casual label. If you want a clean mental shortcut, think of:

• First trimester as roughly months 1–3
• Second trimester as roughly months 4–6
• Third trimester as roughly months 7–9

If you share updates with family, giving both can cut confusion: “I’m 18 weeks, so I’m in the second trimester.” It keeps the math out of the conversation.

How trimester boundaries affect care schedules

Most tests and scans are tied to gestational week ranges. Trimester labels are used as a quick handle, but the scheduling is week-based. ACOG’s patient education on fetal growth gives a sense of how development changes by stage and why certain checks tend to land when they do. You can read it at ACOG’s fetal growth FAQ.

Here’s how trimester thinking often shows up in real planning:

• Early pregnancy: confirm pregnancy, set dating, review history, and talk through screening options.
• Mid pregnancy: anatomy review, growth tracking, and planning for work, travel, and daily comfort.
• Late pregnancy: watch growth and position, review labor signs, and plan for delivery logistics.

If your pregnancy is labeled high-risk, you may have extra checks that don’t fit neatly into a trimester script. That’s normal. The week count stays the anchor.

Common terms you’ll see next to trimester labels

Apps and visit notes use a mix of terms that can sound like code. This table translates the words you’ll see most often, so you can read a portal message without pausing to search every phrase.

Term	What it means	Where it shows up
Gestational age	Weeks counted from the first day of the last period	Visit notes, ultrasound reports, lab ranges
Dating ultrasound	Scan used to set or confirm week count and due date	Early visits when cycle dates are uncertain
EDD	Estimated due date, often around 40 weeks	Charts, appointment printouts, portals
Fundal height	Measure from pubic bone to top of uterus	Second and third trimester visits
Viability	Term used when discussing chances of survival before term	Specialist notes, preterm labor counseling
Term	Phrase used for late pregnancy timing near the due date	Delivery planning, induction conversations
Postterm	Pregnancy continuing beyond 42 weeks in many definitions	Extra monitoring plans, timing talks

A simple way to place yourself on the trimester map

If you want a one-step check, use this:

1. Find your gestational week number from your last period or your clinic’s due date.
2. Match it to the split: 1–12 first, 13–27 second, 28–birth third.
3. If a source uses a different cutoff, ignore the label and follow the week window for the test or milestone you care about.

That’s it. When in doubt, lean on the week count and the date in your medical record.