Pregnancy triggers a cascade of changes that affect nearly every system in your body, from your blood volume to your bone structure. It lasts about 40 weeks, divided into three trimesters, and each one brings distinct shifts for both you and the developing baby. Here’s what actually happens, stage by stage.
Conception and the First Days
Pregnancy begins when a sperm fertilizes an egg, usually in the fallopian tube. The fertilized egg then travels toward the uterus and implants into the uterine lining about six days after fertilization. Once implanted, the embryo starts producing a hormone called hCG, which is the hormone pregnancy tests detect. hCG becomes measurable in your blood around 10 to 11 days after conception, which is why most home tests work best after a missed period.
How Your Hormones Shift
Three hormones drive most of what you feel during pregnancy. hCG rises rapidly in the first trimester and is closely linked to nausea and vomiting. Progesterone thickens the uterine lining to support the embryo and later helps maintain the pregnancy through the placenta. Estrogen, normally produced by the ovaries, is also made by the placenta during pregnancy to support fetal development and keep the pregnancy stable.
A fourth hormone, relaxin, loosens your muscles, joints, and ligaments so your body can stretch as the baby grows. It’s especially active around your pelvis, back, and abdomen. Relaxin is helpful for delivery, but it also makes you less stable on your feet and more prone to sprains. Many people notice changes in their posture or a feeling of wobbliness, particularly later in pregnancy.
What Happens in the First Trimester
The first trimester covers weeks 1 through 12, and it’s when the most dramatic early development takes place. By week 8, the brain and spine have begun forming, cardiac tissue is developing, and the lungs are starting to build the tubes that will eventually carry air. All of this happens while the embryo is still smaller than a grape.
For you, the first trimester often feels like the hardest stretch symptom-wise. Common experiences include:
- Extreme tiredness that goes beyond normal fatigue
- Nausea with or without vomiting, often called morning sickness but possible at any hour
- Tender, swollen breasts
- Frequent urination
- Mood swings
- Constipation and heartburn
- Food cravings or aversions
Most of these symptoms ease as you move into the second trimester.
The Second Trimester: A Relative Break
Weeks 13 through 28 are often described as the most comfortable stretch of pregnancy. Nausea and fatigue typically fade. Your energy returns, and many people feel more like themselves again. This is also when you’ll likely start showing and feeling the baby move for the first time, usually somewhere between weeks 16 and 25.
Behind the scenes, your body is working harder than it feels. Your blood volume increases significantly during pregnancy, typically rising about 45% above pre-pregnancy levels, though the range can be anywhere from 20% to 100%. Your heart pumps more blood per minute to supply the placenta and growing baby. By 24 weeks, your cardiac output (the total amount of blood your heart moves) can be up to 45% higher than before pregnancy. Early on, your heart achieves this by pumping more blood per beat. Later, it compensates by beating faster, eventually adding 10 to 20 extra beats per minute by the third trimester.
The Third Trimester and Preparing for Birth
From week 29 onward, the baby is gaining weight rapidly and the organs are maturing. The lungs are among the last to fully develop. Your body, meanwhile, is carrying significantly more weight, and the effects of relaxin on your pelvis and joints are at their peak. Back pain, difficulty sleeping, shortness of breath, and swelling in the feet and ankles are all common. Your heart rate is now 20% to 25% above your pre-pregnancy baseline.
Weight Gain During Pregnancy
How much weight you gain depends on your starting weight. For overweight individuals (BMI of 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. If you’re carrying twins, the ranges are higher: 37 to 54 pounds for normal-weight individuals, 31 to 50 for overweight, and 25 to 42 for those with obesity. This weight isn’t just baby. It includes the placenta, amniotic fluid, increased blood volume, breast tissue, and fat stores your body builds to support breastfeeding.
Nutrients That Matter Most
Prenatal vitamins fill gaps that are hard to cover through diet alone. Three nutrients deserve particular attention. Folate (600 micrograms daily) supports the development of the baby’s brain and spinal cord, and deficiency early in pregnancy raises the risk of neural tube defects. Iron needs jump to 27 milligrams per day because of the massive increase in blood volume. Calcium requirements are 1,000 milligrams for adults 19 to 50, and 1,300 milligrams for those under 19, to support the baby’s developing bones without depleting your own.
What Labor Looks Like
Labor happens in three stages. The first stage begins with regular contractions that gradually open the cervix. In early labor, the cervix dilates to less than 6 centimeters. During active labor, it opens from 6 to 10 centimeters. Before a vaginal delivery, the cervix needs to be fully thinned out and dilated to 10 centimeters. This first stage is usually the longest part of the process.
The second stage is the actual birth. Pushing can take anywhere from a few minutes to a few hours, depending on factors like the baby’s position, your energy, and whether this is a first delivery. The third stage is the delivery of the placenta, which typically happens within 30 minutes after the baby is born.
Warning Signs to Take Seriously
Most pregnancy symptoms are uncomfortable but normal. A few, however, signal something potentially dangerous. The CDC identifies these as urgent warning signs that need immediate medical attention:
- A headache that won’t go away or gets worse over time, especially with blurred vision or dizziness
- Vision changes like seeing flashes of light, bright spots, or temporary blindness
- Extreme swelling of the hands or face, such as not being able to bend your fingers or open your eyes fully
- Fever of 100.4°F or higher
- Severe belly pain that is sharp, sudden, or worsening
- Trouble breathing or a feeling of tightness in your chest or throat
- Chest pain or a fast, irregular heartbeat with dizziness or faintness
- Severe nausea and vomiting where you can’t keep fluids down for more than 8 hours
- Thoughts of harming yourself or your baby, including persistent sadness, hopelessness, or intrusive thoughts
Several of these, particularly the combination of headaches, vision changes, and facial swelling, can be signs of preeclampsia, a blood pressure condition that develops during pregnancy and can become life-threatening without treatment.