The second trimester starts at 14 weeks and 0 days of pregnancy. So if you’re counting from the very beginning of your pregnancy (the first day of your last menstrual period), you’ll reach the second trimester after completing 13 full weeks and 6 days. How many weeks you have left depends on where you are now, but the finish line is the same for everyone: week 14.
Why Some Sources Say 13 Weeks and Others Say 14
You’ll find apps and websites that place the start of the second trimester at 12, 13, or 14 weeks, which is confusing when you’re trying to pin down a number. The American College of Obstetricians and Gynecologists (ACOG) defines the first trimester as lasting from the first day of your last period through 13 weeks and 6 days. The second trimester then begins at 14 weeks and 0 days and runs through 27 weeks and 6 days.
The math behind this is simple but slightly awkward. Pregnancy lasts about 40 weeks, and dividing that evenly into three trimesters would give you roughly 13 weeks and 3 days each. Since trimesters aren’t perfectly equal thirds, organizations round to clean cutoffs. ACOG draws the line at the end of week 13, making the second trimester begin on the first day of week 14.
The other source of confusion is that pregnancy dating starts about two weeks before you actually conceive. Those first two weeks count toward gestational age even though fertilization hasn’t happened yet. So when someone says they’re “14 weeks pregnant,” the embryo itself is closer to 12 weeks old. This doesn’t change the trimester cutoff, but it explains why the timeline can feel off.
What’s Happening at the Transition
The shift from first to second trimester isn’t just a calendar milestone. Several real changes in your body line up around this time. By week 12, the placenta takes over as the primary source of progesterone and estrogen, a job previously handled by a temporary structure called the corpus luteum. This hormonal handoff is one reason the second trimester often feels so different from the first.
Your uterus also reaches a physical turning point. At about 12 weeks, it grows large enough to rise out of the pelvic cavity and into the abdomen, where it can be felt just above the pubic bone. For many people, this is when a small bump first becomes visible, though that varies with body type and whether it’s a first pregnancy.
At 13 weeks, the fetus measures roughly 68 millimeters (about 2.7 inches) from crown to rump, according to international growth standards from the University of Oxford. All major organs have formed by this point. The second trimester shifts focus from organ formation to rapid growth and development.
When First Trimester Symptoms Ease Up
If you’re asking how many weeks until the second trimester, there’s a good chance you’re counting down the days until nausea fades. Morning sickness typically starts around week 6, peaks near week 10, and begins improving by week 14. That timeline lines up almost perfectly with the trimester boundary, which is why the second trimester has a reputation as the most comfortable stretch of pregnancy.
The relief isn’t instant for everyone. About 90% of people see their nausea resolve by 20 weeks, meaning a small percentage carry it well into the second trimester. But most people notice a real difference in energy and appetite somewhere between weeks 14 and 16.
The hormone behind much of that early misery, hCG, peaks between weeks 8 and 11 and then declines and levels off for the rest of pregnancy. As hCG drops and the placenta stabilizes hormone production, fatigue and nausea tend to follow it down. Many people describe the second trimester as a window of feeling more like themselves again.
Time-Sensitive Screenings Before Week 14
One practical reason to know exactly when the second trimester starts: certain prenatal screenings have a hard deadline. The nuchal translucency scan, an ultrasound that measures fluid at the back of the baby’s neck to screen for chromosomal conditions, must be done between 11 weeks and 13 weeks and 6 days. Once you cross into week 14, the window closes because the measurement becomes unreliable.
If you haven’t had this scan and you’re approaching the end of week 13, it’s worth making sure it’s scheduled before the cutoff. Other first trimester blood tests are often drawn at the same appointment. Second trimester screening options are different and typically happen between weeks 15 and 20, so there’s no gap in care if you miss the earlier window, but the combined first trimester screen is considered more accurate when both parts are completed on time.
A Quick Reference by Current Week
If you want a straightforward answer based on where you are right now:
- Currently 6 weeks: about 8 weeks until the second trimester
- Currently 8 weeks: about 6 weeks to go
- Currently 10 weeks: about 4 weeks to go
- Currently 12 weeks: about 2 weeks to go
- Currently 13 weeks: less than 1 week to go
The second trimester runs from week 14 through week 27, covering roughly the fourth, fifth, and sixth months of pregnancy. After that, the third trimester begins at 28 weeks and carries you through to delivery around week 40.