Pregnancy transforms nearly every system in your body, from your heart and lungs to your skin and digestion. Some changes are obvious, like a growing belly, but many happen invisibly: your blood volume increases by roughly 45%, your heart works significantly harder, your ligaments loosen, and your metabolism speeds up to support a developing baby. Here’s what’s actually happening inside your body across all three trimesters and beyond.
The Hormones Driving Everything
Three hormones do most of the heavy lifting during pregnancy. Human chorionic gonadotropin (hCG) is produced almost exclusively by the placenta and surges during the first trimester. It’s the hormone pregnancy tests detect, and it’s also likely responsible for first-trimester nausea and vomiting. Progesterone, produced first by the ovaries and later by the placenta, thickens the uterine lining so a fertilized egg can implant and grow. Estrogen, also made by the placenta during pregnancy, helps maintain the pregnancy and supports fetal development.
These hormones don’t just affect the uterus. They ripple outward into your cardiovascular system, your digestive tract, your skin, and your joints. Most of the changes described below trace back to one or more of these three hormones rising to levels your body has never experienced before.
Your Heart and Blood Work Much Harder
One of the most dramatic shifts happens in your cardiovascular system. Total blood volume increases by about 45% above pre-pregnancy levels, though the range can be anywhere from 20% to 100% depending on the person. Your body produces this extra blood to supply the placenta and uterus while still meeting your own needs.
Your resting heart rate climbs progressively throughout pregnancy, rising 10 to 20 beats per minute by the third trimester. That’s a 20% to 25% increase over your baseline. Cardiac output, the total amount of blood your heart pumps per minute, can increase by up to 45% by week 24 in a singleton pregnancy. The peak comes during labor and immediately after delivery, when cardiac output jumps 60% to 80% above pre-labor levels. This is why pregnancy can feel physically taxing even when you’re sitting still: your heart is doing significantly more work every minute of the day.
Breathing and Metabolism Speed Up
Your body’s oxygen demands rise steadily as the baby, uterus, and your own organs require more fuel. By the end of pregnancy, you consume about 20% more oxygen than you did before conceiving. Carbon dioxide production rises even more steeply, increasing roughly 35% above non-pregnant levels. Your basal metabolic rate also climbs, which is part of why you feel warmer and need more calories as pregnancy progresses.
Many pregnant people notice they feel short of breath, especially in the third trimester. This partly reflects the growing uterus pressing upward against the diaphragm, but it also reflects your respiratory system working harder to meet those increased metabolic demands.
Digestion Slows Down Considerably
Progesterone relaxes smooth muscle throughout the body, and your digestive tract is lined with smooth muscle. The result: food moves more slowly through your stomach, small intestine, and large intestine. This sluggish transit is the root cause of several classic pregnancy symptoms, including constipation and heartburn. When food sits in the stomach longer, acid is more likely to push upward into the esophagus, especially as the uterus grows and presses against the stomach from below.
The gallbladder is affected too. It empties more slowly during pregnancy, which raises the chances of gallstone formation. Morning sickness in the first trimester is linked to rising hCG levels, while the constipation and heartburn that often worsen in the second and third trimesters are more directly tied to progesterone’s muscle-relaxing effects and the physical pressure of a growing uterus.
Joints and Posture Shift
A hormone called relaxin, produced by both the ovaries and the placenta, loosens your muscles, joints, and ligaments to prepare your body for delivery. The loosening is most pronounced in the pelvis, lower back, and abdomen. This is helpful when the time comes to give birth, but it comes with trade-offs throughout pregnancy.
Loosened pelvic and spinal ligaments can change your posture, often creating an exaggerated curve in the lower back as your center of gravity shifts forward with a growing belly. Many people feel less stable on their feet, and the increased joint laxity makes you more susceptible to sprains and injuries. A belly support band can help distribute weight and improve posture during the second and third trimesters. If you exercise during pregnancy, it’s worth knowing that your joints won’t feel as secure as usual, so high-impact or balance-heavy activities carry more risk than they normally would.
Skin Changes and Hyperpigmentation
Rising hormone levels trigger the placenta to produce melanocyte-stimulating hormone, which increases melanin production throughout your body. Melanin is the pigment that gives skin its color, and more of it means certain areas darken noticeably. The line running from your belly button downward (called the linea nigra) is one of the most visible examples. The same mechanism darkens the areolas around the nipples and can cause melasma, the brownish patches that sometimes appear on the face.
Stretch marks are another common skin change, developing as the skin stretches rapidly to accommodate a growing belly, breasts, and hips. These typically appear as reddish or purple lines during pregnancy and fade to a lighter color afterward, though they rarely disappear entirely.
How Much Weight to Expect
Weight gain during pregnancy isn’t just about the baby. It includes the placenta, amniotic fluid, increased blood volume, larger breasts, a bigger uterus, and additional fat stores. The CDC’s current recommendations, based on pre-pregnancy BMI, are:
- Underweight (BMI under 18.5): 28 to 40 pounds
- Normal weight (BMI 18.5 to 24.9): 25 to 35 pounds
- Overweight (BMI 25.0 to 29.9): 15 to 25 pounds
- Obese (BMI 30.0 to 39.9): 11 to 20 pounds
For twin pregnancies, the ranges are considerably higher. A person with a normal pre-pregnancy BMI carrying twins would be expected to gain 37 to 54 pounds. Most weight gain happens in the second and third trimesters, with the first trimester contributing relatively little, especially if nausea limits appetite.
What Happens After Delivery
The changes don’t reverse overnight. Your uterus begins shrinking almost immediately after birth, but the full process of returning to its pre-pregnancy size takes up to six weeks. Hormones shift dramatically within hours of delivery, which can cause noticeable sweating and mood changes. The sweating typically subsides within a week or two as your body begins recalibrating.
Swelling from the extra fluid you accumulated during pregnancy usually resolves within the first week postpartum as your body sheds that retained water. The cardiovascular system gradually returns to baseline over several weeks, though the exact timeline varies. Skin changes like the linea nigra and melasma often fade in the months after delivery, and joint laxity from relaxin slowly resolves as hormone levels drop. Some changes, like wider hips or abdominal muscle separation, can take longer to fully recover or may persist to some degree permanently.