How Many Weeks Is the Second Trimester in Pregnancy?

The second trimester of pregnancy is generally defined as weeks 13 through 27, though some medical organizations use weeks 14 to 27.

If you’ve looked up how many weeks the second trimester lasts, you’ve probably seen different answers. Cleveland Clinic says weeks 13 to 27. ACOG says 14 weeks and 0 days to 27 weeks and 6 days. Even reputable sources don’t quite agree.

The honest answer is that most experts place the second trimester somewhere between week 13 and week 27, with small variations by source. This article walks through the different definitions, what happens during these weeks, and what you can expect from your own pregnancy timeline.

Standard Definition: Weeks 13 to 27

The second trimester is the middle stretch of pregnancy, typically described as lasting from week 13 through week 27. Cleveland Clinic and the NHS both use this range, and March of Dimes also starts at week 13.

ACOG defines it a bit differently: starting at 14 weeks and 0 days and ending at 27 weeks and 6 days. The difference is only a few days, but it explains why pregnancy apps and doctor’s offices sometimes seem to disagree.

University of Utah Health narrows the window further, listing weeks 14 through 26. For most practical purposes, though, the second trimester runs from roughly week 13 to week 27 — about 14 weeks total, spanning the fourth, fifth, and sixth months.

Why the Exact Weeks Vary by Source

Medical organizations use slightly different criteria for dividing pregnancy into trimesters. This isn’t an error — it’s a matter of convention. Here’s how the major sources line up:

• Cleveland Clinic: Weeks 13 through 27. This is one of the most commonly cited ranges.
• ACOG: 14 weeks 0 days to 27 weeks 6 days. The extra precision comes from counting pregnancy from the last menstrual period.
• University of Utah Health: Weeks 14 through 26. A slightly shorter window used by some academic centers.
• NHS and March of Dimes: Both start at week 13, consistent with the broader definition.
• Tommy’s (UK charity): Spans roughly months 4, 5, and 6 — weeks 13 to 28 in their count.

In practice, the variation is just a few days or a week. Your healthcare provider will use their own preferred system, so it’s perfectly fine to follow whichever guide matches your doctor’s approach.

Fetal Development and Body Changes in the Second Trimester

The second trimester is a time of rapid growth for the baby. By week 13, all major organs are formed and continue to mature. The nervous system develops steadily, and by week 27 the baby’s reflexes and lung structures are still refining.

University of Utah Health’s second trimester timeline notes that during weeks 13 to 16 the baby’s skin forms and bones harden. The startle reflex appears, and air sacs in the lungs begin developing.

Many women experience what clinicians call the “honeymoon period” — morning sickness often fades, energy returns, and a “pregnancy glow” with improved skin and hair is common. You may notice swollen gums, mild belly aches, and the first flutters of fetal movement, usually between weeks 16 and 22.

Most pregnancy apps and countdowns default to the week-13 start. If your provider says you’re in your second trimester, they likely mean weeks 13 to 27, but it’s always okay to ask for their specific definition.

Common Second Trimester Milestones

Knowing where you fall in the second trimester helps you anticipate key events. These are some of the most notable developments:

1. Quickening (feeling baby move): First fetal movements are usually felt between weeks 16 and 22. Earlier for some, later for others.
2. Anatomy scan: This detailed ultrasound is typically scheduled around weeks 18 to 22. It checks the baby’s organs, limbs, and growth.
3. Pregnancy glow: Increased blood flow and hormonal shifts can make skin and hair appear fuller and brighter. Not everyone experiences it.
4. Relief from morning sickness: Nausea often eases by week 14 or 15, and appetite returns for many women.
5. Visible baby bump: The uterus expands above the pubic bone, and the belly becomes noticeably rounder.

These milestones aren’t universal — every pregnancy has its own rhythm. Some women feel movement earlier or later, and symptoms like swelling or leg cramps can appear anytime.

How Many Months Is the Second Trimester?

The second trimester covers roughly months 4, 5, and 6 of pregnancy. That’s about 14 weeks, though calendar months aren’t exactly four weeks long, so the mapping is approximate.

The NHS second trimester guide starts at week 13, which falls in the beginning of month 4. By week 27, you’re finishing month 6 and preparing to enter the third trimester.

If you’re converting weeks to months, here’s a quick breakdown: month 4 is weeks 13 to 16, month 5 is weeks 17 to 20, and month 6 is weeks 21 to 27. Your provider will track by weeks rather than months, so don’t worry if the two don’t line up perfectly.

The Bottom Line

Most experts agree the second trimester runs from roughly week 13 through week 27. A few organizations start at week 14 or end at week 26, but the difference is minor. For daily pregnancy tracking, the week-13 start is the most common and easiest to follow.

Your obstetrician or midwife will use their own timeline based on your due date and early ultrasound measurements. If you’re ever unsure which weeks count, ask at your next appointment — they can match the definition to your specific pregnancy calendar.