A pregnancy is the roughly 40-week process in which a fertilized egg develops into a baby inside the uterus. It begins when a sperm cell joins with an egg cell, and it ends with birth. Those 40 weeks are divided into three trimesters, each with distinct changes for both the developing baby and the pregnant person’s body.
How Pregnancy Starts
Pregnancy begins with conception, the moment a sperm fertilizes an egg. This typically happens in the fallopian tubes within 24 hours of ovulation. Once the egg is fertilized, the resulting cell (called a zygote) starts dividing as it travels toward the uterus. Two cells become four, four become eight, and by about a week later the cluster has grown to roughly 100 cells.
Around six days after fertilization, this cluster attaches to the lining of the uterus in a process called implantation. Implantation is the true starting line: until the embryo embeds in the uterine wall, the pregnancy can’t progress. Once it does, the body begins producing a hormone called hCG, which signals the ovaries to keep making progesterone. Progesterone thickens the uterine lining and supports the growing embryo. A home pregnancy test works by detecting hCG in urine, which is why most tests aren’t reliable until around the time of a missed period.
How Pregnancy Is Dated
One of the most confusing things about pregnancy is how it’s counted. Doctors don’t start the clock at conception. Instead, they count from the first day of your last menstrual period (LMP), which is typically about two weeks before you actually conceived. That means when you’re “four weeks pregnant,” the embryo has only existed for about two weeks.
This method assumes a regular 28-day cycle with ovulation on day 14, which doesn’t apply to everyone. Only about half of women accurately recall the date of their last period, and in one study, 40% of women who received a first-trimester ultrasound had their due date adjusted by more than five days because the ultrasound didn’t match LMP-based dating. Ultrasound in the first trimester is considered the most accurate way to confirm how far along a pregnancy is. The standard due date is set at 280 days (40 weeks) from the LMP, but only about 5% of babies arrive on that exact date.
The Three Trimesters
First Trimester: Weeks 1 Through 13
The first trimester is when all major organs begin to form. By the end of this period, the embryo (called a fetus from about week 10 onward) has developing limbs, eyelids, genitals, and a liver. The kidneys start producing urine, and the pancreas begins making insulin. Cartilage for the hands and feet appears, though it won’t harden into bone for several more weeks.
For the pregnant person, this trimester often brings nausea, fatigue, and breast tenderness. hCG levels rise rapidly, roughly doubling every two to three days in early weeks and peaking somewhere between weeks 8 and 12 at levels that can reach over 200,000 µ/L. These surging hormones are largely responsible for the nausea many people experience. Calorie needs don’t increase much during this stage. About 1,800 calories per day is the general recommendation for most normal-weight pregnant people in the first trimester.
Second Trimester: Weeks 14 Through 27
The second trimester is a period of rapid growth. The fetus develops fingernails, gains the ability to move noticeably, and its organs continue to mature. Many people start to feel fetal movement (often described as fluttering or “quickening”) between weeks 16 and 20. This is also when the pregnancy becomes visibly apparent, as the uterus expands above the pelvic bone.
For many, this trimester feels physically easier than the first. Nausea often fades as hCG levels drop from their peak. Daily calorie needs increase to about 2,200 calories, reflecting the growing demands of the fetus and placenta. Blood volume starts climbing significantly during this period, and cardiac output (how much blood the heart pumps per minute) can increase by up to 45% compared to pre-pregnancy levels. This is one reason some people feel warmer or notice their heart beating harder.
Third Trimester: Weeks 28 Through 40
The third trimester is about weight gain and organ maturation. The fetus puts on most of its birth weight during these final months, and its lungs, brain, and digestive system prepare for life outside the uterus. By week 37, the pregnancy is considered “early term,” and by 39 weeks it’s “full term.”
Physically, this is often the most demanding trimester. The uterus presses against the diaphragm, making breathing feel more effortful. Oxygen consumption rises measurably above pre-pregnancy levels. Total blood volume can increase by 45% or more compared to before pregnancy, placing extra demand on the heart and circulatory system. Calorie needs rise to about 2,400 per day. Swelling in the feet and ankles, back pain, and difficulty sleeping are common as the body adapts to carrying significantly more weight.
What Your Body Does Differently
Pregnancy doesn’t just grow a baby. It fundamentally reshapes the pregnant person’s physiology. The cardiovascular system undergoes some of the most dramatic changes. Blood volume increases by roughly 45% above pre-pregnancy levels, though individual variation is wide, ranging from a 20% to a 100% increase. The heart pumps harder and faster to circulate all that extra blood, reaching up to 45% more output by the 24th week.
Breathing changes too. Even at rest, a pregnant person uses more oxygen than they did before pregnancy, because the body is supplying two circulatory systems. The hormonal environment shifts dramatically: progesterone, which keeps the uterine lining intact and prevents contractions, stays elevated throughout pregnancy. Estrogen also rises steadily and helps increase blood flow to the uterus. These hormonal shifts affect nearly every system, from digestion (which slows down, contributing to heartburn and constipation) to the immune system (which adjusts to avoid rejecting the fetus).
Pregnancies That Develop Differently
Not every pregnancy follows the typical path. In an ectopic pregnancy, the fertilized egg implants outside the uterus, most often in a fallopian tube. This type of pregnancy can’t result in a viable birth and requires medical treatment, because a growing embryo in the fallopian tube can cause life-threatening bleeding.
A molar pregnancy is a rare complication where the cells that would normally form the placenta grow abnormally instead. There are two types: a complete molar pregnancy, where no fetal tissue develops at all, and a partial molar pregnancy, where some abnormal fetal tissue may form alongside the unusual placental growth. The risk of having a repeat molar pregnancy after a first one is about 1 in 100.
Multiple pregnancies (twins, triplets, or more) occur when more than one embryo develops at the same time, either from multiple eggs being fertilized or from a single fertilized egg splitting. Multiple pregnancies carry higher risks of preterm birth and other complications and are monitored more closely throughout.
Nutrition During Pregnancy
Calorie needs increase gradually rather than all at once. The common advice to “eat for two” is misleading. During the first trimester, most people don’t need any extra calories beyond their usual intake. The recommended increase is about 300 additional calories per day overall, but this breaks down unevenly: roughly 1,800 calories in the first trimester, 2,200 in the second, and 2,400 in the third for someone who was at a normal weight before pregnancy.
Beyond calories, certain nutrients become especially important. Folic acid helps prevent neural tube defects and is most critical in the earliest weeks, often before someone knows they’re pregnant. Iron supports the expanding blood volume, and calcium helps build fetal bone. Most prenatal vitamins are designed to fill these specific gaps.