Pregnancy lasts about 40 weeks from the first day of your last menstrual period and is divided into three main stages called trimesters. Each trimester spans roughly 12 to 14 weeks and brings distinct changes for both the developing baby and your body. Some providers also recognize a “fourth trimester,” the 12 weeks after birth, as a critical stage of recovery.
First Trimester: Weeks 1 Through 12
The first trimester is when the most dramatic transformation happens at the smallest scale. A single fertilized egg divides into millions of specialized cells that organize into the foundation of every organ system. By week 5, a primitive heart and circulatory system begin to form. By week 6, the neural tube along the baby’s back closes, giving rise to the brain and spinal cord. These early weeks are why prenatal vitamins with folic acid matter so much: the neural tube finishes closing before many people even confirm they’re pregnant.
By the end of week 12, the embryo (now called a fetus) has recognizable facial features, limb buds that have become tiny arms and legs, and a heartbeat detectable on a doppler. The fetus is only about 2 to 3 inches long, but nearly every major organ has at least started forming.
For you, the first trimester often feels like the hardest stretch physically. A hormone called hCG, produced almost exclusively by the placenta, surges dramatically during these weeks and is closely linked to the nausea and vomiting many people experience. Progesterone rises too, thickening the uterine lining and triggering fatigue, breast tenderness, and mood shifts. Your blood volume starts increasing, which can cause lightheadedness.
First Trimester Screenings
Early prenatal visits typically include blood work, a dating ultrasound, and a conversation about genetic screening. If you opt for it, noninvasive prenatal testing (a blood draw that screens for chromosomal conditions) can be done in the first trimester. Carrier screening for inherited conditions is also offered around this time.
Second Trimester: Weeks 14 Through 27
The second trimester is often described as the most comfortable stretch of pregnancy. Nausea usually fades, energy returns, and the baby bump becomes visible. Behind the scenes, this is a period of rapid growth and development.
Between weeks 17 and 20, the part of the brain that controls motor movements is fully formed, the digestive system starts working, and the ears, nose, and lips become clearly recognizable on an ultrasound. Hearing begins to develop, meaning the fetus can start responding to sounds from outside the womb. In female fetuses, the uterus and vagina begin forming during this window as well.
Somewhere between weeks 16 and 25, most people feel fetal movement for the first time, a milestone called “quickening.” First-time parents tend to notice it later (closer to weeks 20 to 22), while those who’ve been pregnant before often recognize the fluttery sensations earlier. Movement becomes more consistent and stronger as the trimester progresses.
The Anatomy Scan and Glucose Test
Two major appointments happen in the second trimester. The anatomy ultrasound, done between weeks 18 and 22, is a detailed scan of the baby’s organs, spine, brain, and limbs. This is typically when you can learn the sex if you want to. Later, between weeks 24 and 28, you’ll do a glucose tolerance test to screen for gestational diabetes, a condition that affects how your body processes sugar during pregnancy and can influence the baby’s growth.
Third Trimester: Weeks 28 Through 40
The final trimester is all about finishing touches and preparation for life outside the womb. The fetus gains significant weight, building up fat stores that will help regulate body temperature after birth. Lung development accelerates as the lungs begin producing surfactant, a substance that keeps the tiny air sacs from collapsing and is essential for breathing independently. The brain grows rapidly, developing the folds and connections that support basic functions like temperature regulation and coordination.
Between weeks 32 and 36, most babies shift into a head-down position in preparation for delivery. The ideal position is head down, facing your back, with the chin tucked to the chest. Most fetuses settle into this position by week 36. If a baby remains breech (feet or bottom down), your provider will discuss options as your due date approaches.
For you, the third trimester brings increasing physical demands. Your blood volume peaks at about 40 to 50 percent above its pre-pregnancy level. The growing uterus pushes against the bladder, diaphragm, and stomach, causing frequent urination, shortness of breath, and heartburn. Braxton Hicks contractions (irregular tightening of the uterus) become more noticeable and are a normal part of the body preparing for labor.
What “Full Term” Actually Means
Not all deliveries after 37 weeks are considered equal. The American College of Obstetricians and Gynecologists breaks it down like this:
- Early term: 37 weeks through 38 weeks, 6 days
- Full term: 39 weeks through 40 weeks, 6 days
- Late term: 41 weeks through 41 weeks, 6 days
- Post-term: 42 weeks and beyond
This distinction matters because babies born even a week or two early have higher rates of breathing difficulty and feeding trouble compared to those born at 39 weeks or later. It’s why most providers avoid elective inductions or scheduled cesareans before 39 weeks unless there’s a medical reason.
The Fourth Trimester: The First 12 Weeks After Birth
The 12 weeks after delivery are increasingly recognized as a distinct stage of the pregnancy journey. Your body is recovering from one of the most physically demanding events it can go through, and the hormonal shifts are just as dramatic as those of early pregnancy.
Physical recovery looks different depending on how you delivered. After a vaginal birth with tearing, the tissue needs time to heal and should be evaluated at a postpartum visit. After a cesarean, the incision and internal stitches require monitoring. Either way, uterine cramping, bleeding (lochia), and general soreness are normal for several weeks.
The emotional landscape shifts significantly too. As many as 80 percent of new mothers experience “baby blues,” a period of sadness and tearfulness that typically peaks 7 to 14 days after birth and resolves on its own. If overwhelming sadness, hopelessness, severe anxiety, or intrusive thoughts persist beyond 10 to 14 days, that may signal postpartum depression rather than the blues.
Postpartum visits are also an opportunity to check that pregnancy-related complications are resolving. Conditions like high blood pressure or gestational diabetes don’t always disappear the moment the baby arrives, and this period is a window to identify longer-term risk factors for heart disease or metabolic conditions.