Pregnancy lasts about 40 weeks, divided into three trimesters, and each one brings distinct changes to your body and your baby’s development. From the first wave of nausea to the final weeks of preparation for labor, here’s a trimester-by-trimester guide to what’s ahead.
First Trimester: Weeks 1 Through 12
The first trimester is when your body ramps up hormone production to support the pregnancy, and you’ll likely feel it. The hormone hCG, produced almost exclusively by the placenta, rises sharply during these weeks and is thought to drive the nausea and vomiting commonly called morning sickness (which can strike at any time of day). Progesterone thickens the uterine lining to support the embryo, while estrogen helps maintain the pregnancy overall. These hormonal surges are behind most of the early symptoms: extreme tiredness, tender and swollen breasts, food cravings or aversions, mood swings, constipation, frequent urination, headaches, and heartburn.
Your baby develops remarkably fast during this stretch. By four to five weeks, the brain, spinal cord, and heart have begun forming, and tiny arm and leg buds appear. The embryo is roughly one-twenty-fifth of an inch long. By eight weeks, all major organs and external body structures have started to form, the heart beats with a regular rhythm, and fingers and toes are taking shape. Your baby is now called a fetus, looks more recognizably human, and measures nearly one inch long. By the end of week 12, nerves and muscles are working together well enough for the baby to make a fist. The eyelids close to protect developing eyes and won’t reopen until around week 28. At about three inches long and almost one ounce, the external sex organs are now visible.
Second Trimester: Weeks 13 Through 27
Many people find the second trimester the most comfortable stretch of pregnancy. The intense fatigue and nausea of the first trimester typically ease, and the baby isn’t yet large enough to cause the aches and breathing difficulties of later months. You’ll likely start to feel the baby move for the first time, often between weeks 16 and 20, initially as light flutters that grow stronger over time.
The major milestone of this trimester is the 20-week anatomy scan, usually performed between weeks 18 and 22. During this detailed ultrasound, a sonographer takes pictures and measurements of the baby’s heart, brain, spine, kidneys, limbs, face, chest, lungs, and digestive organs. They also record the fetal heart rate, check blood flow through the umbilical cord, assess the position of the placenta, measure the amount of amniotic fluid, and examine your cervix. This scan is one of the primary tools for catching potential developmental problems before birth, and it’s often when parents learn the baby’s sex if they want to know.
Between weeks 26 and 28, you’ll have a glucose screening test. You drink a sugary solution, then have your blood drawn to check how your body processes sugar. If the results are outside the normal range, a longer glucose tolerance test follows to determine whether you have gestational diabetes, a condition that can usually be managed with dietary changes and monitoring.
Third Trimester: Weeks 28 Through 40
As the baby grows larger, your body works harder. Backaches become common. The baby’s size and position can make it difficult to find a comfortable resting position, and you may notice heart palpitations, which feel like fluttering or skipped beats. These are usually harmless and happen because the growing uterus slows blood returning to the heart.
You’ll start to feel Braxton Hicks contractions: slight, irregular tightening sensations in the abdomen that come and go. They tend to appear in the afternoon or evening, after physical activity, or after sex, and they grow more frequent as your due date approaches. Braxton Hicks are the body’s way of rehearsing for labor and are generally nothing to worry about. If you experience more than six contractions in an hour and they’re steadily getting stronger, that may signal actual labor.
Baby movements become easier to feel throughout this trimester. Many providers will ask you to do daily “kick counts” in the third trimester, paying attention to how often the baby moves during a set period. A noticeable slowdown in movement is something to flag right away.
Prenatal Visit Schedule
The standard appointment pattern for an uncomplicated pregnancy is once a month from weeks 4 through 28, twice a month from weeks 28 through 36, then weekly from week 36 until birth. At these visits, your provider checks your weight, blood pressure, urine, and the baby’s heart rate, and discusses any symptoms you’re experiencing. The increasing frequency toward the end allows closer monitoring as labor approaches.
Weight Gain and Nutrition
Recommended weight gain depends on your pre-pregnancy BMI. For overweight individuals (BMI 25 to 29.9), the guidelines suggest gaining 15 to 25 pounds total. For those classified as obese, the recommended range is 11 to 20 pounds. Twin pregnancies have higher targets: 37 to 54 pounds for normal-weight individuals, 31 to 50 pounds for overweight individuals, and 25 to 42 pounds for obese individuals. Your provider can tell you where you fall and whether your gain is on track.
One of the most important supplements to start early, ideally before conception, is folic acid. The CDC recommends 400 micrograms daily for anyone who could become pregnant, as it helps prevent neural tube defects in the baby’s brain and spine. Most prenatal vitamins contain 400 to 800 micrograms. A good prenatal vitamin also covers other increased needs like iron, which supports the dramatic expansion of your blood volume during pregnancy.
Exercise During Pregnancy
Staying active is safe and beneficial for most uncomplicated pregnancies. The recommended target is at least 150 minutes of moderate-intensity aerobic activity per week, which can be as simple as brisk walking, swimming, or stationary cycling spread across several days. If you were already doing vigorous exercise before pregnancy, you can generally continue. Strength training is also encouraged.
The activities to avoid are those with a high risk of abdominal impact or falls (contact sports, downhill skiing) and scuba diving, which poses unique pressure-related risks to the baby.
Sleep Positions
In early and mid-pregnancy, your sleep position doesn’t appear to affect the risk of complications. As you move into the third trimester, side sleeping becomes the standard recommendation. Previous research has linked sleeping on the back with a higher risk of stillbirth, reduced fetal growth, low birth weight, and preeclampsia. The concern is that the weight of the growing uterus can compress the major blood vessels that carry blood to and from your lower body. Sleeping on the left side is often specifically recommended because it keeps pressure off these vessels, but either side is considered safe.
If you wake up on your back, don’t panic. Simply roll to your side. A pillow between your knees or behind your back can help you stay in position.
Warning Signs That Need Immediate Attention
Most pregnancy symptoms are uncomfortable but normal. A few, however, require urgent medical care:
- Severe or worsening headache that doesn’t respond to rest or fluids
- Vision changes such as flashes of light, blind spots, blurriness, or double vision
- Extreme swelling of your hands or face, especially if it’s hard to bend your fingers or open your eyes fully
- Fever of 100.4°F or higher
- Trouble breathing, tightness in your chest or throat, or needing to prop yourself up to sleep
- Chest pain or a fast, pounding, or irregular heartbeat
- Severe belly pain that is sharp, sudden, or worsening
- Vaginal bleeding heavier than light spotting, fluid leaking from the vagina, or foul-smelling discharge
- Severe nausea and vomiting where you can’t keep fluids down for more than 8 hours or food for more than 24 hours
- Decreased fetal movement, or the feeling that the baby has stopped moving or is moving less than usual
- Dizziness or fainting that is ongoing or recurrent
- Thoughts of harming yourself or your baby, persistent sadness, hopelessness, or extreme anxiety
These symptoms can signal conditions like preeclampsia, placental problems, or infection, all of which are treatable when caught early. If something feels wrong, err on the side of getting checked.