Your second trimester begins at week 13 of pregnancy and lasts through the end of week 27. That’s roughly three and a half months in the middle stretch of pregnancy, and many people consider it the most comfortable phase. If you’ve been counting the days through first-trimester nausea and exhaustion, this is the transition point most pregnant people look forward to.
You may notice some sources say the second trimester starts at week 14 instead. This comes from a technical distinction in how pregnancies are dated: the measurement used to estimate gestational age in the first trimester (crown-rump length) becomes less accurate beyond about 14 weeks, so some clinical guidelines draw the line there. In practice, the difference is a single week, and most major medical centers use week 13 as the starting point.
How Pregnancy Weeks Are Counted
Pregnancy is dated from the first day of your last menstrual period, not from conception. That means you’re already considered about two weeks pregnant at the time of ovulation and fertilization. So when you reach “13 weeks pregnant,” your baby has actually been developing for roughly 11 weeks. This counting method catches many people off guard, but it’s universal in obstetric care and is how your due date was calculated.
If your periods were irregular or you’re unsure of dates, an early ultrasound (before 14 weeks) is the most accurate way to pin down gestational age, with precision within about five to seven days. After that window, dating estimates widen to plus or minus seven to ten days, based on a combination of measurements like head circumference and thigh bone length.
What Changes in Your Body
The second trimester brings a visible shift. Your uterus grows above the pelvic bone, and a baby bump becomes noticeable to others. Behind the scenes, your blood volume increases by roughly 1.5 liters over the course of pregnancy, with much of that expansion happening now. This extra blood supports the placenta and prepares your body for delivery, but it can also cause stuffy noses, bleeding gums, and occasional dizziness when you stand up quickly.
Your diaphragm shifts upward by about 5 centimeters as the uterus expands, which is why you may start feeling slightly short of breath even during mild activity. This is normal and doesn’t mean your baby is short on oxygen.
Weight gain picks up during this trimester. For someone who started pregnancy at a normal weight, the overall target is 25 to 35 pounds across all three trimesters. People who were underweight before pregnancy are guided toward 50 to 62 pounds total, while those who were overweight typically aim for 15 to 25 pounds. Most of the visible gain happens in the second and third trimesters, so don’t be surprised if the scale starts moving more noticeably now.
When You’ll Feel the Baby Move
One of the most anticipated milestones of the second trimester is “quickening,” the first time you feel your baby move. If this is your first pregnancy, expect to notice it around 20 weeks. If you’ve been pregnant before, you may recognize those flutters as early as 16 weeks. The movements start subtle, often described as bubbles, popcorn popping, or a light tapping. By the end of the second trimester, kicks and rolls become unmistakable.
Key Screenings and Tests
The biggest appointment of the second trimester is the anatomy scan, a detailed ultrasound typically scheduled between weeks 18 and 22. This is the one where the technician spends a long time measuring your baby’s head, abdomen, and thigh bone, checking the brain, spine, heart, kidneys, and other organs, and evaluating the placenta’s position and amniotic fluid levels. It’s also when you can learn the sex, if you want to know.
Between weeks 24 and 28, you’ll be offered a glucose screening test for gestational diabetes. This involves drinking a sugary solution and having your blood drawn about an hour later. If your blood sugar comes back high, a longer follow-up test confirms whether gestational diabetes is present. The condition is manageable but important to catch, since it affects how your baby grows in the third trimester.
How Your Baby Develops
At the start of the second trimester, around week 13, your baby is roughly the size of a lemon. Over the next 15 weeks, development accelerates dramatically. Facial features become more defined, fingerprints form, and the body starts growing hair. Your baby begins swallowing amniotic fluid, which helps the digestive system practice for life outside the womb.
By week 20, the halfway point of pregnancy, your baby’s hearing is developing, and they may start responding to loud sounds. By week 27, the end of the second trimester, the nervous system is actively maturing, the eyes can open and close, and the lungs are beginning to produce the substance they’ll need to breathe air. Babies born at 27 weeks face serious challenges but have increasingly good survival rates with intensive care, a sign of just how much development happens in this trimester.
A Quick Trimester Breakdown
- First trimester: Week 1 through week 12
- Second trimester: Week 13 through week 27
- Third trimester: Week 28 through week 40 (or delivery)
If you’re trying to figure out exactly where you fall, check the gestational age on your most recent ultrasound report or count forward from the first day of your last period. Any week between 13 and 27 puts you squarely in the second trimester.