Pregnancy begins when a sperm fertilizes an egg, and that fertilized egg successfully implants in the uterine wall. From there, it takes about 40 weeks for a single cell to develop into a fully formed baby ready for birth. The process involves a tightly coordinated sequence of cell division, organ formation, and dramatic changes to the pregnant person’s body.
Fertilization: How It Starts
Pregnancy starts in the fallopian tube, not the uterus. Each month, one of the ovaries releases an egg during ovulation. That egg survives for about 24 hours. If sperm are present in the fallopian tube during that window, fertilization can happen.
Getting to the egg is no small feat for sperm. After entering the vagina, sperm undergo a process called capacitation, where the acidic environment triggers changes that boost their motility and energy. Of the millions of sperm released, only a few hundred reach the egg. The first sperm to arrive must break through two protective layers surrounding the egg. The sperm’s tip, called the acrosome, releases enzymes that dissolve these barriers one at a time, allowing it to reach and fuse with the egg’s surface.
The moment one sperm gets through, the egg locks everyone else out. A surge of calcium triggers the egg to release chemicals that harden its outer shell, making it impenetrable to additional sperm. This prevents the embryo from receiving a double dose of genetic material, which would be fatal. At this point, the sperm’s DNA merges with the egg’s DNA, forming a single cell with a complete set of 46 chromosomes. That cell is called a zygote, and it immediately begins dividing.
Implantation: Settling Into the Uterus
Over the next several days, the fertilized egg divides again and again as it travels down the fallopian tube toward the uterus. By the time it arrives, it has become a hollow ball of roughly 100 cells called a blastocyst. About six days after fertilization, the blastocyst attaches to the wall of the uterus, usually near the top. This process, called implantation, is completed by day nine or ten.
Not every fertilized egg makes it this far. Many blastocysts fail to implant and are shed during menstruation without the person ever knowing fertilization occurred. When implantation does succeed, the outer layer of cells begins burrowing into the uterine lining and forming connections with the mother’s blood supply. Some of these cells will become the placenta. Others will become the embryo itself.
How Hormones Keep the Pregnancy Going
Once the embryo implants, the body shifts into a hormonal state designed to protect and sustain the pregnancy. Three hormones do most of the heavy lifting.
Progesterone is the first critical player. Produced initially by the ovaries and later by the placenta, it thickens the uterine lining to nourish the embryo and prevents the uterus from contracting and shedding that lining. Without adequate progesterone, the body would essentially treat the pregnancy like a normal menstrual cycle and shed it.
Human chorionic gonadotropin (hCG) is the hormone pregnancy tests detect. It is produced almost exclusively by the placenta and rises rapidly during the first trimester. Its primary job early on is to signal the ovaries to keep producing progesterone until the placenta is mature enough to take over. Estrogen, also produced by the placenta, supports blood flow to the uterus and helps maintain a healthy pregnancy throughout all three trimesters.
The First Trimester: Building the Blueprint
The first 12 weeks are the most critical for development. This is when all major organs begin to form, a stage called organogenesis. For the first eight weeks after fertilization, the developing life is called an embryo. After that, it’s called a fetus.
During this period, the brain, spinal cord, heart, lungs, liver, and digestive system all take shape. The heart begins beating. Limb buds appear and grow into arms and legs with fingers and toes. Because organ formation is happening so rapidly, the first trimester is the window when the embryo is most vulnerable to disruption from alcohol, certain medications, infections, or nutritional deficiencies. This is why folic acid intake matters so much early on. Pregnant people need 600 micrograms daily, and ideally should start supplementing with at least 400 micrograms a month before conception. Folic acid is essential for proper development of the brain and spinal cord.
The Second Trimester: Growth and Movement
Weeks 13 through 26 are often described as the most comfortable stretch of pregnancy. Morning sickness typically fades, energy returns, and the fetus is growing rapidly but isn’t yet large enough to cause significant physical strain.
This is the trimester when the senses come online. By around 16 weeks, the eyes can move slowly and the ears are nearly in their final position. By 18 weeks, the ears begin picking up sound. The fetus can hear the mother’s heartbeat, digestive sounds, and eventually external voices and music. By 23 weeks, the eyes start making rapid movements during sleep, a sign of developing visual processing.
Around 18 weeks after conception (roughly 20 weeks of pregnancy, since pregnancy is dated from the last menstrual period), many people feel fetal movement for the first time. This sensation, sometimes called quickening, often starts as a subtle fluttering before becoming unmistakable kicks and rolls in later weeks. First-time pregnancies tend to notice it a bit later than subsequent ones, simply because the sensation is unfamiliar.
The Third Trimester: Final Preparation
From week 27 until birth, the fetus is primarily gaining weight, building fat stores, and maturing its organs for life outside the womb. The lungs are among the last organs to fully mature. They need to produce a substance called surfactant, which keeps the tiny air sacs from collapsing after birth. This is one of the main reasons premature babies often struggle with breathing.
The brain also undergoes enormous growth during these final months, forming the complex folds and connections that will support learning, movement, and sensation after birth. The fetus practices breathing movements, swallows amniotic fluid, and cycles between sleep and wakefulness. By the end of the third trimester, an average full-term baby weighs about 6 to 9 pounds.
The Placenta: A Temporary Organ
The placenta is one of the most remarkable parts of pregnancy. It’s a temporary organ, built from scratch every pregnancy and delivered after the baby is born. Its job is to act as a barrier and exchange system between the mother’s blood and the fetus’s blood, which never actually mix.
Inside the placenta, maternal and fetal blood flow in close proximity, separated by a membrane just 2 to 3 micrometers thick. Oxygen passes from the mother’s blood to the fetal blood because of a pressure gradient, much like how air flows from a high-pressure area to a low-pressure area. The fetus has a built-in advantage here: fetal hemoglobin (the molecule in red blood cells that carries oxygen) has a higher affinity for oxygen than adult hemoglobin, so it pulls oxygen across the membrane more efficiently. Carbon dioxide and other waste products flow in the opposite direction, from fetus to mother, for her body to eliminate.
The placenta also delivers nutrients, produces hormones, and provides some immune protection. It is, for all practical purposes, the fetus’s lungs, kidneys, and digestive system rolled into one.
How the Mother’s Body Adapts
Pregnancy doesn’t just grow a baby. It fundamentally reshapes the pregnant person’s physiology. One of the most dramatic changes is in blood volume, which increases by 30 to 50 percent by near full term. This happens through increased fluid retention and higher cardiac output, ensuring that the uterus, placenta, and fetus receive enough blood flow. It’s also why pregnancy can cause swollen ankles, a faster resting heart rate, and feelings of breathlessness even during mild activity.
The uterus itself grows from roughly the size of a pear to the size of a watermelon. Ligaments loosen throughout the body in preparation for delivery. The kidneys filter more blood, metabolism increases, and iron requirements jump to 27 milligrams per day (nearly double the non-pregnant recommendation) to support all that extra red blood cell production.
What “Full Term” Actually Means
Pregnancy is dated from the first day of the last menstrual period, which means “40 weeks pregnant” actually includes about two weeks before conception even occurred. The American College of Obstetricians and Gynecologists defines full term as 39 weeks through 40 weeks and 6 days. Babies born at 37 to 38 weeks are considered early term, while those born at 41 weeks are late term. These distinctions matter because even a week or two of extra development improves outcomes for breathing, feeding, and temperature regulation.