A baby is made when a sperm cell from a male fertilizes an egg cell from a female, forming a single new cell that carries a complete set of genetic instructions. That single cell then divides repeatedly, travels to the uterus, and implants in the uterine wall to begin a pregnancy. The entire process, from sex to a detectable pregnancy, takes roughly two to three weeks. Here’s how each step works.
The Egg: Where It All Starts
Each month, a woman’s body prepares to release an egg. A hormone called follicle-stimulating hormone triggers the growth of 3 to 30 small fluid-filled sacs (follicles) inside the ovaries, each containing an immature egg. One follicle becomes dominant and continues to grow while the others fade away. When a second hormone, luteinizing hormone, surges, the dominant follicle ruptures and releases its egg into the fallopian tube. This moment is ovulation.
The egg survives for about 12 to 24 hours after release. That short window is why timing matters. But because sperm can survive inside the uterus and fallopian tubes for 3 to 5 days, the total fertile window in each cycle is about 6 days: the 5 days before ovulation and the day of ovulation itself. Sex on any of those days can lead to pregnancy.
The Sperm’s Journey
During sex, hundreds of millions of sperm are deposited near the cervix. From there, they face a gauntlet of obstacles. Most are swept back out by fluid flow. Others get trapped in thick cervical mucus, are absorbed by the immune system, or simply swim in the wrong direction. Of the millions that start the journey, only a few hundred reach the fallopian tube where the egg is waiting.
Sperm that do make it travel through the cervix, across the uterus, and into the fallopian tube. The entire distance is only about 15 to 18 centimeters, but at the scale of a sperm cell, it’s an enormous trek through hostile territory. The fastest sperm can reach the fallopian tube within 30 minutes, though many take longer.
Fertilization: Sperm Meets Egg
When sperm reach the egg, they encounter two protective layers. First is a cloud of supporting cells surrounding the egg called the cumulus. Sperm that push through this layer then reach the zona pellucida, a tough outer shell. To get through it, sperm undergo a chemical change that releases enzymes from a cap on their head, allowing them to digest a path through the shell. The first sperm to fully penetrate the zona pellucida and reach the egg’s membrane fuses with it.
Once that first sperm enters, the egg immediately changes its outer shell to block any additional sperm from getting in. This is critical. If two sperm fertilized the same egg, the embryo would have too much genetic material and couldn’t develop normally. Within hours, the genetic material from the sperm and egg combines, and a brand-new cell called a zygote is formed.
How Your Baby Gets Its DNA
Every cell in your body contains 46 chromosomes, arranged in 23 pairs. But egg and sperm cells are special: each carries only 23 chromosomes, one from every pair. When they fuse, the zygote gets 23 chromosomes from the mother and 23 from the father, restoring the full set of 46. This is why children inherit traits from both parents.
The baby’s sex is determined at this exact moment. The mother’s egg always contributes an X chromosome. The father’s sperm carries either an X or a Y. If the sperm delivers an X, the baby will be female (XX). If it delivers a Y, the baby will be male (XY). So biologically, the sperm determines the sex of the baby.
From One Cell to Many
The zygote doesn’t stay a single cell for long. Within about 24 hours, it divides into two cells. Those two become four, then eight, and so on. While dividing, the tiny cluster of cells is slowly traveling down the fallopian tube toward the uterus, pushed along by the gentle waving of hair-like structures lining the tube.
By around day 3 or 4 after fertilization, the cluster has become a solid ball of cells called a morula. By day 5 or 6, it has reorganized into a hollow sphere called a blastocyst. The blastocyst has two distinct parts: an inner group of cells that will become the baby and an outer layer called the trophoblast that will eventually form the placenta.
Implantation: The Pregnancy Begins
Around 6 days after fertilization, the blastocyst reaches the uterus and begins attaching to the uterine wall. The outer trophoblast layer is coated with a sticky protein that binds to carbohydrate molecules on the uterine lining. Researchers at the NIH have described it as a gradual braking process: the blastocyst rolls along the uterine wall, slowing down as more and more of these molecular connections form, until it comes to a complete stop and burrows into the lining.
Once embedded, the blastocyst taps into the mother’s blood supply for nutrients and oxygen. This is the true start of pregnancy. The trophoblast begins producing a hormone called hCG, which signals the body to maintain the uterine lining rather than shedding it as a period. HCG levels build quickly, and in many cases they’re high enough to trigger a positive result on a home pregnancy test about 10 days after conception.
Odds of Conception Per Cycle
Even when everything works perfectly, pregnancy doesn’t happen every time. A woman in her early to mid-20s has roughly a 25 to 30 percent chance of conceiving in any given menstrual cycle. That probability starts to decline slowly in the early 30s and drops more steeply after age 35. This is largely because egg quality and quantity decrease with age, making fertilization and successful implantation less likely over time.
Many fertilized eggs never implant at all, or implant briefly and then stop developing before a woman even knows she’s pregnant. Estimates suggest that a significant portion of conceptions end this way, which is one reason the per-cycle success rate stays well below 100 percent even in young, healthy couples.
How Twins Happen
Twins form in two fundamentally different ways. Fraternal twins occur when a woman releases two eggs during the same cycle and each is fertilized by a different sperm. These twins share about 50 percent of their DNA, just like any other siblings, and can be different sexes.
Identical twins start from a single fertilized egg that splits into two separate embryos during the first few days of development. Because they originate from the same zygote, identical twins share virtually all of their DNA and are always the same sex. Why the split happens isn’t fully understood, but it’s not influenced by family history the way fraternal twinning can be.