Making a baby requires a sperm cell to meet and fertilize an egg cell, which then implants in the uterus and grows into a pregnancy. That’s the short version. The longer version involves precise timing, a chain of biological events, and, for some people, medical assistance. Here’s how it all works.
How Fertilization Happens
Each month, one of the ovaries releases a single egg in a process called ovulation. Tiny finger-like structures guide the egg into the fallopian tube, where it begins traveling toward the uterus. This is where fertilization takes place.
During sex, millions of sperm are released into the vagina and swim up through the cervix and uterus into the fallopian tubes. Despite those millions, only one sperm breaks through the egg’s outer layer to fertilize it. The moment that happens, the egg’s surface changes to block any other sperm from entering.
The fertilized egg, now called a zygote, starts dividing as it continues traveling down the fallopian tube: two cells, then four, then more. About five to six days after fertilization, it has become a ball of roughly 100 cells called a blastocyst. This blastocyst reaches the uterus and, over the next one to three days, burrows into the thickened uterine lining in a process called implantation. Once implantation is complete, pregnancy has officially begun.
The Fertile Window
You can only get pregnant during a roughly six-day window each menstrual cycle. Sperm survive inside the reproductive tract for about three to five days, and the egg remains viable for about 24 hours after ovulation. That means sex up to five days before ovulation or one day after can result in pregnancy.
In a typical 26- to 28-day cycle, ovulation happens around days 12 to 14. For the best chance of conceiving, having sex every day or every other day during that six-day window is what fertility experts recommend. Cycle length varies from person to person, though, so tracking ovulation through symptoms, temperature charting, or ovulation test strips can help pinpoint your personal timing.
What Happens After Implantation
Once the blastocyst implants in the uterine lining, it begins producing a hormone called hCG. This is the hormone that home pregnancy tests detect. In a standard 28-day cycle, hCG typically reaches detectable levels in urine 12 to 15 days after ovulation. Testing before that point can produce a false negative simply because hormone levels haven’t built up enough yet.
Age and Fertility
Age is one of the biggest factors affecting how easily conception happens. Fertility peaks around age 30 for both men and women, then declines at different rates. Women aged 35 to 40 have a 23% lower chance of conceiving in any given cycle compared to women in their early twenties. For men, the decline is smaller: about a 5% reduction for those aged 35 to 39.
The combined effect is significant. If both partners are the same age, their chance of conceiving at 40 is nearly half of what it was at 30. This decline is driven primarily by egg quality and quantity decreasing over time, though sperm quality also changes with age.
Preparing Your Body Before Conception
One of the most important steps you can take before trying to conceive is starting a folic acid supplement. The U.S. Preventive Services Task Force recommends 400 to 800 micrograms daily for anyone planning or able to become pregnant. Ideally, you’d start at least one month before trying to conceive and continue through the first two to three months of pregnancy. Folic acid helps prevent neural tube defects, which are serious problems with the brain and spine that develop very early, often before you even know you’re pregnant.
If you use lubricant during sex, it’s worth knowing that most commercial lubricants, and even saliva, can slow sperm movement. Household oils like coconut oil can also interfere. If you’re trying to conceive, look for lubricants specifically labeled “fertility-friendly” or “sperm-friendly,” which are designed to match natural vaginal mucus without affecting sperm. Avoid products with fragrances or parabens.
When Conception Needs Medical Help
Not everyone conceives through sex alone. Fertility treatments offer alternative paths, with two of the most common being IUI and IVF.
Intrauterine Insemination (IUI)
IUI, sometimes called artificial insemination, places sperm directly into the uterus using a thin catheter. The sperm is first washed and prepared to remove chemicals that could impair fertility. The procedure is timed to coincide with ovulation, often after taking medication that stimulates the ovaries to produce multiple eggs. IUI is less invasive and less expensive than IVF, and it’s a common first step for couples with unexplained infertility or for people using donor sperm.
In Vitro Fertilization (IVF)
IVF is more involved. It also starts with fertility medications to stimulate egg production, but then the eggs are physically retrieved from the ovaries using a fine needle under anesthesia. Those eggs are fertilized with sperm in a lab. In cases of severe sperm-related infertility, a single sperm can be injected directly into each egg. The resulting embryos grow for five to six days until they reach the blastocyst stage, then one is transferred into the uterus. IVF bypasses many of the natural barriers to conception, making it an option when other approaches haven’t worked or when specific medical conditions are involved.
What Sperm Need to Succeed
Healthy sperm aren’t just about quantity. The World Health Organization sets reference values based on men who successfully fathered children. At a minimum, a fertile sample typically contains at least 15 million sperm per milliliter, with at least 39 million total per ejaculate. At least 40% of those sperm need to be moving, and at least 32% should be swimming forward in a purposeful direction. Perhaps surprisingly, only about 4% of sperm need to be normally shaped for fertility to be within the expected range. Even in fertile men, the vast majority of sperm have some structural irregularity.
Low numbers in any of these categories don’t necessarily mean you can’t conceive, but they can make it harder and may point toward treatments like IUI or IVF to improve the odds.