Pregnancy lasts about 40 weeks from the first day of your last menstrual period and is divided into three main stages called trimesters, each roughly 13 weeks long. Every trimester brings distinct changes for both the developing baby and your body. Here’s what happens in each stage, what you can expect to feel, and what comes after delivery.
First Trimester: Weeks 1 Through 12
The first trimester is when the most dramatic biological work happens, even though your belly barely shows. By week 5, a primitive heart and circulatory system begin forming. By week 6, the heart and other organs are taking shape, and tiny buds appear that will become arms. Leg buds follow around week 7. By the end of this trimester, every major organ system has started developing, and the embryo officially becomes a fetus.
Meanwhile, your body ramps up production of progesterone and other hormones to sustain the pregnancy. These hormonal surges are responsible for most of the classic early symptoms: nausea (with or without vomiting), deep fatigue, food cravings or aversions, heartburn, and constipation. Progesterone in particular slows your digestive system and relaxes the valve between your stomach and esophagus, which is why heartburn can start surprisingly early. Most of these symptoms ease as you move into the second trimester, though the timeline varies.
This is also when your first round of prenatal screening typically happens. Genetic carrier testing and early screening through blood tests or ultrasound are offered in the first trimester for those who want them.
Second Trimester: Weeks 13 Through 26
The second trimester is often called the most comfortable stretch of pregnancy. Nausea usually fades, energy returns, and the baby grows rapidly. The major milestone most people look forward to is quickening, the first time you feel the baby move. This typically happens around 18 weeks after conception (roughly 20 weeks of pregnancy), though it can be a few weeks earlier or later, especially in a first pregnancy.
Your body undergoes significant cardiovascular changes during this period. Total blood volume increases by about 45% above pre-pregnancy levels, though it can range from 20% to 100% depending on the individual. Your heart rate also gradually climbs, eventually rising 10 to 20 beats per minute above your baseline. These changes support the growing placenta and baby but can leave you feeling short of breath or lightheaded at times.
A glucose tolerance test is standard between 24 and 28 weeks to screen for gestational diabetes. This involves drinking a sugary solution and having your blood drawn to check how your body processes the sugar. A detailed anatomy ultrasound, usually scheduled around 18 to 22 weeks, checks the baby’s organs, limbs, and growth.
Third Trimester: Weeks 27 Through 40
The final trimester is all about growth and preparation for birth. The baby gains the most weight during these weeks, and its lungs undergo critical maturation. The last key substance needed for independent breathing isn’t produced until after 35 weeks, which is one reason babies born very early often need breathing support. A pregnancy is considered “full term” at 39 weeks, though labor can begin naturally anywhere from 37 to 42 weeks.
You’ll likely start noticing Braxton Hicks contractions, sometimes called practice contractions, during this trimester. They feel like a tightening across your abdomen but stay irregular in timing and intensity. The key difference from real labor contractions is the pattern: true contractions get progressively stronger, longer, and closer together, while Braxton Hicks remain unpredictable and eventually stop on their own.
A vaginal and rectal swab for Group B Strep bacteria is done around 36 to 37 weeks. This common screening determines whether you’ll need antibiotics during labor to protect the baby.
The Three Stages of Labor
Labor itself has three distinct stages, and understanding them helps you know what to expect when the time comes.
Stage 1: Contractions and cervical dilation. This is the longest stage, lasting anywhere from 12 to 19 hours for a first-time birth. It’s divided into early labor, when your cervix opens to about 6 centimeters (typically 6 to 12 hours), and active labor, when dilation reaches a full 10 centimeters (typically 4 to 8 hours). Contractions become increasingly intense and frequent as you move from early to active labor. If you’ve given birth before, this stage often goes much faster.
Stage 2: Pushing and delivery. Once your cervix is fully dilated, pushing begins. This stage can last from 30 minutes to several hours. For first-time births, it’s considered prolonged if it exceeds three hours without an epidural or four hours with one.
Stage 3: Delivery of the placenta. After the baby is born, the placenta still needs to be delivered. This is the shortest stage, usually wrapping up within 30 minutes.
The Fourth Trimester: After Birth
The three months after delivery are increasingly recognized as a distinct stage of their own. During this period, your body reverses many of the changes it spent nine months building. The uterus, which expanded from the size of a fist to an organ holding 13 to 15 pounds of baby, placenta, and amniotic fluid, shrinks back to its pre-pregnancy size in just six to eight weeks.
Hormones shift dramatically after delivery, which can affect mood, sleep, and energy on top of the physical demands of caring for a newborn. Recovery from birth itself, whether vaginal or surgical, is a significant physiological process. The fourth trimester is as much about your own healing as it is about the baby’s adjustment to life outside the womb.
Prenatal Weight Gain Guidelines
How much weight you’re expected to gain depends on your pre-pregnancy BMI. Overweight individuals (BMI 25 to 29.9) are generally recommended to gain 15 to 25 pounds total. For those with a BMI of 30 or higher, the recommended range is 11 to 20 pounds. Your provider will track your weight throughout pregnancy and adjust guidance based on your starting point and how things progress. Most of the weight comes from the baby, placenta, amniotic fluid, increased blood volume, and breast tissue rather than stored fat.