A baby begins when a single sperm cell from a father joins with an egg cell from a mother, forming the very first cell of a new human being. From that moment, it takes about 40 weeks of growth inside the mother’s uterus before the baby is ready to be born. While this article doesn’t include photographs, it walks through every major stage clearly enough that you can picture each one, and searching any stage name below will bring up detailed medical illustrations and ultrasound images.
How Sperm and Egg Meet
A mother’s body releases one egg from an ovary roughly once a month, in a process called ovulation. That egg travels into the fallopian tube, a narrow passage connecting the ovary to the uterus. Sperm can survive inside the mother’s body for three to five days, so conception doesn’t have to happen on the exact day the egg is released. The egg itself, though, is only viable for about 12 to 24 hours after ovulation.
When sperm enter the reproductive tract, fluids inside the mother’s body prime them for fertilization. This process thins the outer membrane on the head of each sperm, preparing it to release digestive enzymes it will need later. Of the millions of sperm that start the journey, only a few hundred typically reach the egg in the fallopian tube. They’re drawn toward it by chemical signals released from the cells surrounding the egg.
Fertilization: Two Cells Become One
The egg is wrapped in two protective layers. The outer layer is a loose ring of cells, and beneath it sits a thicker shell. Some sperm release their enzymes early, helping to dissolve the outer ring so that others can pass through. A single sperm then pushes through the inner shell and makes contact with the egg’s surface, where it binds to special receptors. The membranes of the sperm and egg fuse, and the sperm’s genetic material enters the egg.
The moment that happens, the egg triggers a rapid chemical reaction. It releases proteins that destroy its remaining sperm receptors and coats itself in a barrier no other sperm can penetrate. This ensures only one sperm contributes its DNA. Each parent’s cell carries 23 chromosomes, and when they combine, the fertilized egg (now called a zygote) contains the full set of 46 chromosomes that every human cell needs. This single cell holds every genetic instruction for building a complete person.
From One Cell to a Ball of Hundreds
The zygote begins dividing almost immediately. One cell becomes two, two become four, and so on. Over the next five to six days, as it drifts down the fallopian tube toward the uterus, it transforms into a hollow ball of roughly 200 to 300 cells called a blastocyst. At this point it’s still tiny, smaller than a grain of sand.
The blastocyst has two distinct parts. The outer layer of cells will eventually form the placenta. A small inner cluster of cells will become the baby itself. Once the blastocyst reaches the uterus, it sheds its outer membrane in a process called hatching. Sticky proteins on its surface then bind to the uterine lining, and it burrows in. This is implantation, and it marks the true beginning of pregnancy.
The Placenta: A Lifeline Between Mother and Baby
As the embryo settles into the uterine wall, the outer cells develop into the placenta, an organ roughly the size of a dinner plate by the end of pregnancy. The placenta is filled with tiny finger-like structures called villi, packed with blood vessels. The mother’s blood flows around these villi, delivering oxygen and nutrients while filtering out carbon dioxide and waste from the baby’s blood. Crucially, the mother’s blood and the baby’s blood never actually mix. The placenta acts as a selective barrier, connected to the baby through the umbilical cord.
How the Baby Grows Week by Week
Development in the first trimester is astonishingly fast. By week five, the neural tube, which will become the brain and spinal cord, has already formed. By week five or six, cells that will form the heart begin clustering together and pulsing. That tiny tube-shaped heart beats about 110 times per minute by the end of week five. By week six, a provider can often detect those pulses on an ultrasound, and by week nine, the heartbeat is strong enough to hear with a handheld Doppler device.
During the second trimester, the baby’s organs continue to mature. Around week 15, the lungs start to develop, though the baby practices “breathing” by inhaling and exhaling amniotic fluid rather than air. By week 20, the area of the brain responsible for the five senses begins forming. The baby can respond to sound and light, and many parents feel their first kicks during this period.
The third trimester is largely about growth and final preparation. By week 24, the lungs are developed but not yet ready to function outside the womb. At week 26, the lungs begin producing surfactant, a slippery substance that keeps the air sacs from collapsing after birth. By month nine, the lungs are close to fully developed, and the baby typically weighs six to nine pounds and is positioned head-down, ready for delivery.
How the Baby Is Born
Labor happens in three stages. The first stage begins with contractions, rhythmic tightening of the uterus that gradually opens the cervix (the narrow opening at the bottom of the uterus) to about four inches wide. This stage can last anywhere from a few hours to more than a day, especially for a first birth.
The second stage is active pushing. The mother bears down with each contraction to move the baby through the birth canal. Crowning is the moment the top of the baby’s head becomes visible, and shortly after, the baby is fully born. The third stage is delivering the placenta, which separates from the uterine wall and is pushed out. The umbilical cord is then cut, leaving what will become the baby’s belly button.
When Conception Happens in a Lab
Not all babies are conceived through intercourse. In vitro fertilization, or IVF, combines sperm and egg outside the body. Eggs are collected from the mother’s ovaries after a course of fertility medications designed to produce multiple mature eggs at once. Those eggs are then mixed with sperm in a laboratory dish. If fertilization succeeds, the resulting embryo grows in a controlled environment for several days until it reaches the blastocyst stage. A doctor then transfers the blastocyst into the mother’s uterus, where it can implant just as it would in a natural pregnancy. From that point forward, the pregnancy proceeds the same way.
Where to Find Visual Guides
Medical sites like the Cleveland Clinic and Mayo Clinic publish detailed illustrations and week-by-week photos of fetal development. Searching “fetal development images by week” will show medically accurate visuals for every stage described above. Many pregnancy apps also offer 3D renderings that update weekly, giving you a realistic sense of the baby’s size and appearance at each milestone. For younger children asking how babies are made, illustrated books designed for specific age groups can present this information in a way that’s both accurate and appropriate.