When you’re pregnant, your body begins a massive transformation almost immediately. Hormones surge, your blood volume increases by nearly 45%, and a single fertilized cell develops into a fully formed baby over roughly 40 weeks. Some of these changes are obvious, like a growing belly and morning sickness. Others are invisible but equally dramatic, reshaping your cardiovascular system, digestion, joints, and immune function from the inside out.
The First Few Weeks: Hormones Take Over
Pregnancy starts with a hormonal cascade. About a day after the fertilized egg implants in your uterine wall (around eight days after ovulation), your body begins producing a hormone called hCG. This is the hormone that turns a pregnancy test positive. Its main job is to keep the corpus luteum, a temporary structure in your ovary, alive and pumping out progesterone. Without that progesterone, the uterine lining would shed and the pregnancy would end.
hCG rises fast, reaching a peak of about 100,000 IU/L in your blood between weeks 8 and 10. After that, it drops to a lower, steady level for the rest of pregnancy. Meanwhile, progesterone does far more than maintain the uterine lining. It dials down your immune system’s response to the embryo so your body doesn’t reject it, and it relaxes smooth muscle throughout your body. That relaxation effect is why pregnancy affects digestion, blood pressure, and even your veins. By the time you reach full term, your placenta is producing roughly 250 milligrams of progesterone per day.
First Trimester Symptoms
The most noticeable early symptoms tend to show up between weeks 4 and 9. Nausea, commonly called morning sickness, can hit at any time of day. Your breasts may feel swollen and tender as hormonal changes ramp up blood flow and begin preparing milk ducts. Fatigue is one of the earliest and most universal symptoms, driven largely by rising progesterone levels.
You might also notice more frequent urination, food aversions, or a heightened sense of smell. These symptoms vary widely from person to person and even from one pregnancy to the next. For most people, nausea and fatigue improve significantly by the start of the second trimester around week 13.
How Your Baby Develops
Fetal development moves remarkably fast. By week 8, all of the major organs and body systems have started forming. The embryo has tiny, webbed hands and feet, visible eyes, and the beginnings of ears. The umbilical cord is fully functional, transporting oxygen and nutrients. At this point, the embryo is about the size of a black bean, roughly half an inch to one inch long.
By week 12, all organs, limbs, bones, and muscles are present. The circulatory, digestive, and urinary systems are working. The liver is producing bile, and the fetus is actually swallowing and urinating amniotic fluid. It’s about 2.5 to 3 inches long, roughly the size of a plum.
At week 20, the halfway point, the brain region responsible for the five senses starts to develop and fingernails are growing toward the tips of the fingers. The fetus weighs about one pound and measures 9 to 10 inches. This is also when many people first feel fetal movement, a milestone called quickening. If you’ve been pregnant before, you may notice it a few weeks earlier.
By week 32, the skin is no longer translucent and most organs are well formed and nearly ready for life outside the womb. The lungs and brain are still maturing. At this stage the fetus weighs around 5 pounds and measures 17 to 18 inches.
Your Cardiovascular System Works Harder
One of the most dramatic shifts in pregnancy is how much harder your heart works. By just eight weeks, cardiac output has already increased by 20%. Over the full pregnancy, it rises by about 40%. Your blood volume climbs by roughly 45%, adding 1,200 to 1,600 milliliters above your normal levels. Most of that increase happens by week 34.
This extra blood supports the placenta, the growing uterus, and the fetus, but it also affects you directly. Your blood vessels dilate to handle the extra volume, which drops your blood pressure in the first and second trimesters. That dip in blood pressure is why some pregnant people feel dizzy when standing up quickly. It typically normalizes or rises slightly in the third trimester.
Digestion Slows Down
Progesterone relaxes smooth muscle everywhere, including the muscles that move food through your digestive tract. The result is slower digestion. Food sits in your stomach and intestines longer, which can cause bloating, gas, and constipation. The valve between your esophagus and stomach also relaxes, making it easier for stomach acid to travel upward. That’s why heartburn and acid reflux are so common in pregnancy, particularly as the uterus grows and pushes up against the stomach in the second and third trimesters.
Eating smaller, more frequent meals and staying upright after eating can help. Constipation often improves with extra water and fiber.
Joints and Ligaments Loosen
Your body produces a hormone called relaxin, primarily from the corpus luteum, that loosens ligaments and tendons throughout pregnancy. Relaxin levels are highest during the first three months and again near delivery. Its primary target is the pelvis, where it softens the ligaments connecting your pelvic bones to make room for birth.
The trade-off is that this loosening isn’t limited to the pelvis. It affects ligaments and tendons throughout your body, which is why pregnant people are more prone to ankle sprains, knee instability, and lower back pain. The combination of loosened joints and a shifting center of gravity as the belly grows explains why balance and comfort change so much in the later months.
What the Second Trimester Feels Like
Many people call the second trimester the most comfortable stretch of pregnancy. First trimester nausea and exhaustion usually fade, and the belly hasn’t yet grown large enough to cause significant discomfort. Energy levels often rebound.
This is also when pregnancy becomes more visible and more tangible. You’ll likely start showing, and feeling the baby move for the first time around week 20 is a landmark moment. Prenatal visits during this period typically happen every four weeks. These appointments track the baby’s growth, check your blood pressure, and screen for conditions like gestational diabetes.
The Third Trimester: Preparing for Birth
The final stretch brings a new set of physical demands. The uterus is now large enough to press against your lungs, making deep breaths harder, and against your bladder, sending you to the bathroom constantly. Sleep becomes more difficult as finding a comfortable position gets challenging.
Many people experience Braxton Hicks contractions starting in the second or third trimester. These feel like a tightening across your abdomen, similar to mild menstrual cramps. They’re irregular, don’t get progressively stronger, and usually stop if you change positions or walk around. Real labor contractions, by contrast, come at regular intervals, last 30 to 90 seconds each, get stronger and closer together over time, and don’t go away when you move. True labor may also come with your water breaking or a bloody, mucus-like discharge.
Prenatal visits increase in frequency during this period: every two weeks in the seventh and eighth months, then weekly until delivery.
Weight Gain and Nutrition
How much weight gain is expected depends on your pre-pregnancy BMI. For overweight individuals (BMI 25 to 29.9), guidelines recommend gaining 15 to 25 pounds total. For those with a BMI of 30 or higher, the recommendation is 11 to 20 pounds. Normal-weight individuals generally fall in the 25 to 35 pound range.
Your nutritional needs shift during pregnancy. Iron requirements jump to 30 milligrams per day, nearly double the non-pregnant recommendation, because your body is producing so much extra blood. Iron deficiency anemia is one of the most common pregnancy complications. Folic acid is critical in the earliest weeks for proper development of the baby’s brain and spinal cord, which is why it’s recommended even before conception. Calcium supports the baby’s developing bones and teeth, and if you don’t get enough, your body will pull it from your own skeleton.
Warning Signs to Take Seriously
Most pregnancy symptoms are uncomfortable but normal. A few, however, signal something potentially dangerous. Preeclampsia, a condition involving dangerously high blood pressure, can develop after week 20 and produces distinctive symptoms: a severe headache that won’t respond to medication, vision changes like flashing lights or blind spots, and extreme swelling of the hands or face. This isn’t the mild ankle swelling that’s common in late pregnancy. It’s swelling severe enough that you can’t bend your fingers or your eyes look puffy.
Severe pain, redness, or swelling in one leg, especially in the calf, can indicate a blood clot. Pregnancy increases clotting risk because of changes to blood volume and circulation. Any sudden, one-sided leg swelling that’s warm to the touch warrants immediate attention.