Making a baby requires a sperm cell to fertilize an egg, which then implants in the uterus and develops over roughly 40 weeks into a full-term pregnancy. That process can happen through sex or through assisted reproduction like IVF. Here’s how each step works, from egg release to a positive pregnancy test.
How the Body Releases an Egg
Each month, the ovaries prepare a group of eggs for potential release. During days 10 to 14 of the menstrual cycle, one egg matures fully inside a small fluid-filled sac called a follicle. Around day 14 (in a typical 28-day cycle), a surge of a hormone called luteinizing hormone triggers the ovary to release that mature egg. This moment is ovulation, and it’s the starting point for conception.
The released egg travels into one of the two fallopian tubes, where it survives for about 12 to 24 hours. If sperm is already present or arrives during that window, fertilization can happen. If not, the egg breaks down and is shed with the uterine lining during the next period.
The Fertile Window
You don’t have to time sex to the exact hour of ovulation. Sperm can survive inside the body for three to five days, waiting in the cervix, uterus, and fallopian tubes for an egg to appear. This creates a fertile window of roughly seven days: the five days before ovulation, the day of ovulation, and the day after. For the best chances of pregnancy, having sex every day or every other day during that window is what Johns Hopkins Medicine recommends.
Tracking ovulation can be done with over-the-counter test strips that detect the luteinizing hormone surge in urine, by monitoring changes in cervical mucus, or by tracking basal body temperature each morning. None of these methods are perfectly precise, which is why covering several days around the expected ovulation date improves your odds.
What Happens During Fertilization
During ejaculation, millions of sperm cells enter the vagina and swim upward through the cervix and uterus into the fallopian tubes. Despite those enormous numbers, only a few hundred actually reach the egg. Of those, just one sperm breaks through the egg’s outer layer and fuses with it. The moment that happens, the egg’s surface changes to block any other sperm from entering.
The fertilized egg, now called a zygote, begins dividing almost immediately. Within a few hours it becomes two cells, then four, then eight. Over the next five to six days, it continues dividing as it slowly travels down the fallopian tube toward the uterus. By the time it arrives, it has become a hollow ball of about 200 to 300 cells called a blastocyst.
Implantation in the Uterus
The blastocyst floats in the uterus for a brief period before attaching to the uterine lining. For implantation to happen, the blastocyst sheds its protective outer membrane in a process called hatching, which takes one to three days after it enters the uterus. Once hatched, cells on its outer surface release a sticky protein that binds to the uterine lining, anchoring the embryo in place.
After implantation, the embryo begins producing a hormone called hCG, which signals the body to maintain the uterine lining instead of shedding it. Rising hCG levels are what pregnancy tests detect. Home tests can pick up hCG in urine about 12 to 15 days after ovulation, though the FDA recommends testing one to two weeks after a missed period for the most reliable results.
How Likely Is Conception Each Month?
Even with well-timed sex, pregnancy doesn’t happen every cycle. A woman in her early to mid-20s has a 25 to 30 percent chance of conceiving in any given month. That probability holds relatively steady through the late 20s, then gradually declines. By age 40, the chance drops to around 5 percent per cycle. The decline is driven mostly by egg quality and quantity, both of which decrease with age.
For most couples under 35, it takes an average of several months of trying before pregnancy occurs. If you’ve been trying for 12 months without success (or six months if you’re over 35), that’s generally the point where fertility evaluation becomes worthwhile.
How IVF Works
When natural conception isn’t possible or hasn’t worked, in vitro fertilization offers an alternative path. IVF bypasses the fallopian tubes entirely by combining eggs and sperm in a laboratory.
The process starts with ovarian stimulation: injectable hormone medications encourage multiple eggs to mature at once rather than the usual single egg per cycle. Over about 10 to 14 days, ultrasounds and blood tests track how the ovaries are responding. When the eggs are ready, a “trigger shot” finalizes their maturation.
During egg retrieval, a thin needle guided by ultrasound passes through the vaginal wall into each ovary and suctions out the mature eggs. The procedure is brief and done under sedation. That same afternoon, an embryologist injects a single sperm cell directly into each mature egg. Over the next five to six days, the fertilized eggs are monitored as they develop. On average, about half of fertilized eggs reach the blastocyst stage suitable for transfer.
The embryo transfer itself is a simple procedure that doesn’t require anesthesia. A thin catheter passes through the cervix, and one embryo is placed into the uterus. A blood test about 9 to 14 days later confirms whether the embryo implanted successfully. Success rates per complete IVF cycle (including any frozen embryo transfers from the same round) are roughly 43 percent for women aged 30 to 34, 31 percent for ages 35 to 39, and 11 percent for ages 40 to 44.
From Implantation to Pregnancy
Once the embryo implants, whether through natural conception or IVF, the timeline is the same. The embryo develops rapidly: a heartbeat is detectable by about six weeks, and all major organs begin forming during the first trimester (weeks 1 through 12). The second trimester brings noticeable growth as the fetus develops bones, muscles, and the ability to move. By the third trimester, the baby gains weight, the lungs mature, and the body prepares for birth. Full-term delivery typically happens around 40 weeks from the first day of the last menstrual period.
The earliest signs of pregnancy, before a test confirms it, often include a missed period, breast tenderness, nausea, and fatigue. These are driven by the same hormonal shifts that sustain the pregnancy, particularly rising hCG and progesterone levels in the weeks after implantation.