Pregnancy lasts about 40 weeks, divided into three trimesters, and each one brings distinct physical changes, new symptoms, and major milestones in your baby’s development. Your body will increase its blood volume by roughly 45%, grow an entirely new organ (the placenta), and shift its hormone balance dramatically. Knowing what’s coming at each stage helps you tell the difference between what’s normal and what needs attention.
First Trimester: Weeks 1 Through 12
The first trimester is when pregnancy hormones ramp up fast, and your body starts adapting even before you look pregnant. A hormone called hCG, produced almost exclusively by the placenta, rises steeply during these early weeks. It’s largely responsible for the nausea and vomiting that affect a majority of pregnant people, commonly called morning sickness despite often lasting all day. The most common early symptoms include missed periods, nausea, breast tenderness, fatigue, and frequent urination.
Fatigue in the first trimester can be surprisingly intense. Progesterone levels climb to thicken the uterine lining and maintain the pregnancy, and one side effect is deep, persistent tiredness that sleep doesn’t always fix. Many people describe the first trimester as the hardest emotionally because the symptoms are strong but nothing is visible yet, which can feel isolating.
Meanwhile, your baby’s development is moving at a remarkable pace. Cardiac tissue begins forming in the earliest weeks, and the foundations of the lungs, specifically the tubes that will eventually carry air in and out, start taking shape. By the end of the first trimester, all major organ systems have begun to develop.
Second Trimester: Weeks 13 Through 27
Most people feel noticeably better in the second trimester. Nausea typically fades as hCG levels plateau, energy returns, and the pregnancy becomes visible. Your heart is now pumping significantly more blood. By 24 weeks, cardiac output can increase by up to 45% above pre-pregnancy levels to support the growing placenta and baby. That extra blood flow is why you may notice a stuffy nose, bleeding gums, or visible veins you’ve never seen before.
Between weeks 17 and 20, the part of your baby’s brain controlling motor movement is fully formed, and you’ll likely start feeling those first flutters of movement. These early kicks can feel like bubbles or a light tapping. As the weeks progress, movement becomes stronger and more predictable. By the end of this trimester, between weeks 25 and 28, your baby can respond to loud sounds with a startle reflex, pulling arms and legs inward. The lungs are fully formed by this point but not yet ready to function outside the uterus. They’re beginning to produce surfactant, a substance essential for breathing after birth.
Common second-trimester symptoms include round ligament pain (sharp tugging sensations on the sides of your belly as the uterus grows), heartburn, backaches, and skin changes like a dark line appearing down your abdomen. Some people develop mild swelling in the feet and ankles as blood volume continues to rise.
Third Trimester: Weeks 28 Through 40
The final stretch is physically demanding. Your baby is gaining weight rapidly, pressing on your bladder, lungs, and ribs. Shortness of breath, frequent urination (again), difficulty sleeping, and Braxton Hicks contractions, which are irregular practice contractions that tighten your belly, are all common. Your total blood volume increase by this point ranges from 20% to 100% above pre-pregnancy levels, though it typically settles around 45%.
Sleep becomes harder for practical reasons: your belly makes many positions uncomfortable, and the need to urinate can wake you several times a night. Sleeping on your side is generally recommended in late pregnancy, particularly after 32 weeks. Research has linked going to sleep on your back in the third trimester with a higher risk of stillbirth, likely because the weight of the uterus compresses major blood vessels. A pillow between your knees or behind your back can help you stay on your side comfortably.
In the final weeks, you may notice your baby “dropping” lower into your pelvis, which relieves some pressure on your lungs but increases pressure on your bladder. Braxton Hicks contractions may become more frequent. True labor contractions differ because they come at regular intervals, get progressively stronger, and don’t go away when you change positions.
Prenatal Tests and Screenings
You’ll have regular checkups throughout pregnancy, but certain tests happen at specific windows. In the first trimester, blood work and sometimes an early ultrasound establish your due date and check for certain conditions. Some people opt for noninvasive prenatal testing (NIPT), a blood draw that screens for chromosomal differences, available as early as 10 weeks.
The second trimester brings additional screening between 15 and 20 weeks. The anatomy ultrasound, usually done around 18 to 20 weeks, is when a provider checks all the major structures of your baby’s body: brain, heart, spine, kidneys, and limbs. This is also the appointment where you can find out the sex if you want to. A glucose screening, typically done between 24 and 28 weeks, checks for gestational diabetes. You’ll drink a sugary solution and have your blood drawn an hour later.
In the third trimester, appointments become more frequent, often every two weeks and then weekly as your due date approaches. These visits track your blood pressure, your baby’s position, and growth.
Weight Gain Guidelines
How much weight you’re expected to gain depends on your pre-pregnancy BMI. For a single pregnancy, the general recommendations are:
- Normal weight (BMI 18.5 to 24.9): 25 to 35 pounds total
- Overweight (BMI 25 to 29.9): 15 to 25 pounds
- BMI of 30 or higher: 11 to 20 pounds
If you’re carrying twins, the numbers are higher: 37 to 54 pounds for normal weight, 31 to 50 pounds for overweight, and 25 to 42 pounds for a BMI of 30 or above. Most of this weight isn’t body fat. It includes the baby, placenta, amniotic fluid, increased blood volume, and breast tissue growth. Weight gain is usually minimal in the first trimester and accelerates in the second and third.
Caffeine, Exercise, and Daily Adjustments
Caffeine doesn’t need to be eliminated, but keeping intake under 200 milligrams per day is the widely accepted guideline. That’s roughly one 12-ounce cup of coffee. At this level, caffeine does not appear to be a major contributing factor in miscarriage or preterm birth. Keep in mind that tea, chocolate, and some sodas contribute to your daily total.
Moderate exercise is encouraged throughout pregnancy for most people. Walking, swimming, and prenatal yoga are popular choices. The general goal is about 150 minutes of moderate activity per week, meaning you can talk but not sing during the exercise. Activities with a high fall risk, contact sports, or anything done at high altitude or in excessive heat should be avoided. If you were active before pregnancy, you can usually continue your routine with modifications as your belly grows.
Warning Signs That Need Immediate Attention
Many uncomfortable symptoms during pregnancy are normal. Some are not. The CDC identifies several urgent warning signs that require immediate medical care during pregnancy or up to a year after delivery:
- Severe headache that won’t go away, especially with blurred vision or dizziness
- Vision changes like flashes of light, bright spots, or sudden blurry or double vision
- Extreme swelling in your face or hands, to the point where you can’t open your eyes or bend your fingers
- Severe belly pain that is sudden, sharp, or persistent
- Vaginal bleeding beyond light spotting, or fluid leaking from the vagina
- Decreased fetal movement or a noticeable change in your baby’s usual pattern of kicks
- Fever of 100.4°F or higher
- Chest pain, pressure, or a fast or irregular heartbeat
- Severe nausea and vomiting where you can’t keep fluids down
- Leg pain with swelling, redness, or warmth, particularly in one calf, which could signal a blood clot
- Thoughts of harming yourself or your baby, or feeling hopeless or extremely anxious
Several of these, particularly the combination of headache, vision changes, and extreme swelling, are hallmarks of preeclampsia, a blood pressure condition that can escalate quickly. If something feels wrong, it’s always safer to get checked than to wait.
The First Weeks After Birth
The postpartum period, sometimes called the fourth trimester, catches many people off guard. Your body just did something enormous, and recovery takes time. Vaginal bleeding (lochia) continues for four to six weeks after delivery, gradually lightening in color and flow. If you’re soaking through a pad every hour for several consecutive hours, that’s a sign of postpartum hemorrhage and needs immediate evaluation.
Hormones shift dramatically after delivery, and as many as 80% of new mothers experience what’s commonly called the “baby blues,” a period of sadness, tearfulness, and mood swings lasting roughly 7 to 14 days. This is driven by the sudden drop in pregnancy hormones combined with sleep deprivation and the sheer adjustment of caring for a newborn. Baby blues are common and temporary. Symptoms that last longer than two weeks, feel increasingly severe, or include thoughts of harming yourself or your baby are signs of postpartum depression or anxiety, which are treatable conditions that affect a significant number of new parents.
During labor itself, your heart works harder than at any other point in pregnancy. Cardiac output spikes 60% to 80% above pre-labor levels. This is one reason the postpartum period requires genuine rest. Your cardiovascular system, pelvic floor, abdominal muscles, and hormonal balance all need weeks to recalibrate. Recovery isn’t linear, and the six-week postpartum checkup is a starting point for evaluation, not a finish line.