How Many Weeks Are in the First Trimester? | Weeks 1-13

The first trimester is generally defined by most U.S. health authorities as lasting 13 weeks, from week 1 through week 13 of pregnancy.

When you first hear that pregnancy lasts 40 weeks, it’s natural to wonder how those weeks divide into three trimesters. Most people assume each trimester is about 13 to 14 weeks, but the first one seems to get a different number depending on where you look.

The honest answer is that the first trimester is generally considered to last from week 1 through week 13 of pregnancy — a total of 13 weeks. Some health organizations, especially outside the U.S., define it as weeks 1 through 12. The variation comes down to how gestational age is calculated and when the second trimester is considered to begin.

The Standard Definition of the First Trimester

The American College of Obstetricians and Gynecologists (ACOG) defines the first trimester as the period from the first day of your last menstrual period (LMP) through 13 weeks and 6 days. That means you’re considered about 2 weeks pregnant at the time of conception, since LMP is used as the starting point.

Most U.S. hospitals and academic medical centers follow this 13-week definition. For example, Johns Hopkins Medicine, Cleveland Clinic, and NewYork-Presbyterian all describe the first trimester as weeks 1 through 13.

Outside the U.S., some sources like Tommy’s (UK) and the Better Health Channel (Australia) define the first trimester as weeks 1 through 12. This isn’t wrong — it’s just a different convention. The key is to know which definition your healthcare provider uses.

Why the Number of Weeks Can Feel Confusing

Pregnancy dating doesn’t follow calendar months, which is why the first trimester length can seem inconsistent. Here are the main reasons for the discrepancy.

• Starting point confusion: Pregnancy dating begins from the last menstrual period, not from conception, so you’re already “2 weeks pregnant” at fertilization.
• Four-week months: A textbook pregnancy month has 4 weeks, so 3 months would be 12 weeks, but trimesters don’t align perfectly with calendar months.
• Provider preference: Some OB-GYNs shift to the second trimester at 12 weeks, while others wait until 13 weeks — both are common.
• Embryo vs. fetus timing: The transition from embryo to fetus at 8-9 weeks can make the first trimester feel like two distinct phases.
• Different health systems: UK and Australian guidelines often use 12 weeks, while U.S. guidelines use 13 — neither is incorrect.

For most pregnancy apps, calculators, and doctor visits in the U.S., 13 weeks is the standard. If you’re unsure, ask your provider when they consider the first trimester to end.

How Development Maps to First Trimester Weeks

Major organ development happens rapidly during these early weeks. A guide from the Louisiana Department of Health tracks fetal growth each week — the Louisiana first trimester page maps milestones from conception to 13 weeks, noting that by week 13 the fetus measures about 2.5 inches and weighs 0.5 ounces.

These milestones happen quickly in the first weeks. Understanding the timeline helps you know what to expect during prenatal visits and why early prenatal care is important.

Common First Trimester Symptoms and Milestones

The first trimester is often described as the hardest part of pregnancy due to rapid hormonal changes. Many women experience a combination of the following symptoms.

1. Nausea and vomiting: Often called “morning sickness,” it can occur at any time of day and typically peaks around weeks 6-9 before easing.
2. Fatigue: Rising progesterone levels can cause extreme tiredness, making it hard to get through the day without rest.
3. Breast tenderness: Increased blood flow and hormonal shifts make the breasts feel sore, heavy, or tingly.
4. Food aversions and cravings: Many women develop strong dislikes for certain smells or tastes, or unusual cravings.
5. Mood swings: Rapid changes in estrogen and progesterone can lead to emotional ups and downs.

These symptoms usually improve by the second trimester. If they become severe, especially nausea that prevents eating or drinking, contact your provider.

How the First Trimester Fits Into the Full Pregnancy Timeline

The first trimester is the shortest of the three, lasting 13 weeks compared to 14 weeks for the second and 13 weeks for the third (from week 28 to 40). Hopkins explains the transition in its first trimester pregnancy page, noting that the 13-week mark is when many women start to feel better and the risk of miscarriage drops.

Your healthcare provider will track your progression through these trimesters with regular checkups. Prenatal vitamins, especially folic acid, are most critical in the first trimester to support neural tube development.

The Bottom Line

So, how many weeks are in the first trimester? For most U.S. healthcare providers, the answer is 13 weeks — from week 1 through week 13 of pregnancy. Some sources define it as 12 weeks, so it’s always a good idea to confirm with your own OB-GYN or midwife which timeline they use.

Whether you’re counting weeks on an app or following your provider’s schedule, knowing the first trimester covers weeks 1 through 13 helps you track key milestones like organ development and the drop in miscarriage risk. Your obstetrician can guide you on what prenatal care and tests to expect during these critical early weeks.