What Week Does the 2nd Trimester Start? | Weeks 13-27

The second trimester is clinically defined as beginning at week 13 (13 weeks 0 days) and lasting through the end of week 27 (27 weeks 6 days).

You probably know pregnancy is divided into three trimesters, roughly three months each. But the exact week the second trimester starts can feel surprisingly fuzzy, especially when different calculators, apps, or even healthcare providers give slightly different answers.

The standard answer, used by most major medical institutions, is that the second trimester begins at week 13 and lasts through week 27. A smaller number of sources define the start as week 14, which explains the conflicting information you might see. This article walks through the clinical standard, what to expect for fetal development, and why opinions on trimesters can vary.

The Clinical Standard: Week 13

When medical guidelines talk about the second trimester, they consistently point to week 13 as the starting line. The Cleveland Clinic and the NHS both officially define the second trimester as beginning at 13 weeks and 0 days. This is the most widely used framework in prenatal care.

At this point, the risk of miscarriage drops significantly compared to the first trimester, which is one reason many women feel a sense of relief entering this stage. The placenta has fully taken over hormone production, which can help decrease nausea and fatigue for many women.

By week 13, the fetus is starting to make small, random movements. Bones begin to harden, and major organ systems are in place. The second trimester marks a shift from basic organ formation to a long period of growth and maturation.

Why The Confusion About Dates Exists

You may have seen a due date calculator or a pregnancy book that labels week 14 as the start of the second trimester. Why the discrepancy? It usually comes down to how pregnancy is grouped for counting purposes, with some systems rounding to exact 13-week blocks.

• Standard medical definition: Most major hospitals and health organizations (Mayo Clinic, Cleveland Clinic, NHS) define the second trimester as starting at week 13. This is the framework for most prenatal testing schedules.
• Nausea relief timeline: Many women find their energy levels improve and morning sickness eases around week 13 or 14, which can make either date feel like the real start. The Cleveland Clinic refers to this period as the honeymoon phase for that reason.
• Fetal viability milestones: By week 13, the fetus has major organ systems in place and is moving spontaneously. The start of the second trimester marks a shift to a growth and strength phase.
• Minority view (Week 14): Some sources, such as Corewell Health, define the second trimester as starting at week 14. This is less common but worth noting if you are comparing different pregnancy guides.

In practice, your OB-GYN or midwife will most likely use week 13 as the marker when discussing screening tests, anatomy scans, and fetal development milestones.

What’s Happening With Your Baby

The second trimester is a rapid growth period. Here is a look at some of the key fetal development milestones that happen between weeks 13 and 27.

Per the NHS second trimester weeks guide, the fetus begins to make random movements and the bones start to harden. It is a busy time for development across all major systems.

The second trimester is also when you will likely have your anatomy scan, usually around week 20. This ultrasound checks fetal growth and organ development and can often reveal the sex of the baby.

What You Might Experience

Many women describe the second trimester as the most comfortable phase of pregnancy. The fatigue and nausea that marked the first trimester often begin to fade, though individual experiences vary. Here are some common experiences women report during weeks 13 through 27.

1. Energy returns: With the placenta managing hormones, many women report feeling more like themselves. The Cleveland Clinic notes this is why the second trimester is often called the honeymoon period.
2. Physical changes become visible: Your belly will likely begin to show more clearly as the uterus expands to accommodate the growing fetus.
3. Fetal movement becomes noticeable: Around weeks 18 to 22, many women begin to feel the baby kick and move. The Hopkins Medicine guide confirms the fetus can kick, move, and turn from side to side during this period.
4. Skin and hair changes: Hormonal shifts can lead to changes in skin pigmentation and hair texture, which some women notice during the second trimester.
5. Appetite and digestion: Many women find their appetite returns to normal or increases, though heartburn may begin to appear as the uterus expands.

Tracking your symptoms week by week can help you feel more prepared for the changes ahead.

The Week 14 Distinction

As mentioned earlier, a minority of sources define the second trimester as starting at week 14. This is based on a different method of grouping the 40 weeks of pregnancy.

For example, the Louisiana Department of Health provides fetal size at 14 weeks, noting the baby is about 5 inches long and weighs around 8 ounces by that point. The week-14 date is simply a different way of grouping, not a cause for concern.

Whether you consider the start to be week 13 or week 14 has no impact on your prenatal care. Your provider will schedule anatomy scans, glucose tests, and checkups based on your specific dates, not a rigid week number.

The Bottom Line

The second trimester is clinically defined as starting at week 13 of pregnancy. This is the standard used by the NHS, Cleveland Clinic, Mayo Clinic, and most prenatal care frameworks. Some sources define it as week 14, but week 13 is the clinical marker you can confidently rely on for understanding your pregnancy timeline.

Your obstetrician or midwife will calculate your exact dates and trimester transitions based on your last menstrual period or early ultrasound, so you can always ask them for the timeline specific to your pregnancy.