People make kids when a sperm cell fertilizes an egg cell, creating the first cell of a new human being. This can happen through sex, or with medical help in a clinic. Either way, the basic biology is the same: sperm meets egg, the fertilized egg implants in the uterus, and a pregnancy begins.
How Fertilization Works
During sex, millions of sperm travel from the vagina up through the uterus and into the fallopian tubes. These are the two narrow tubes connecting the ovaries to the uterus, and they’re where fertilization almost always happens. An egg released from the ovary during ovulation waits there, and sperm race to reach it. Out of millions, only one sperm breaks through the egg’s outer layer to fertilize it.
Once fertilized, the egg is called a zygote. It immediately starts dividing: two cells, then four, then more. Over the next week, it travels down the fallopian tube toward the uterus, growing into a ball of about 100 cells called a blastocyst. That cluster then attaches to the lining of the uterus in a process called implantation, which typically happens between six and ten days after fertilization. Implantation is the moment a pregnancy truly begins, because the embryo starts drawing nutrients from the parent’s body.
Two hormones, estrogen and progesterone, prepare the uterine lining to receive the embryo. The lining thickens and develops special surface changes that allow the embryo to latch on. If implantation doesn’t happen, the lining sheds during a menstrual period.
The Fertile Window
Pregnancy can only result from sex during a narrow window of about six days per menstrual cycle: the five days before ovulation and the day of ovulation itself. Sperm can survive inside the body for up to five days, which is why sex even a few days before the egg is released can lead to pregnancy. The egg, however, is only viable for roughly 12 to 24 hours after ovulation. After that day, the fertile window closes until the next cycle.
For someone with a typical 28-day cycle, ovulation usually occurs around day 14. But cycles vary, so the exact timing shifts from person to person and even month to month.
How Age Affects the Chances
Age plays a significant role in how quickly pregnancy happens. A large North American study tracked couples actively trying to conceive and found clear differences by age group. Among women aged 25 to 27, about 79% became pregnant within 12 months of trying. That number held fairly steady through the early 30s, with women aged 31 to 33 reaching about 77%.
The decline becomes sharper after 35. Women aged 37 to 39 had a 67% chance of conceiving within a year, and for those 40 to 45, the probability dropped to roughly 56%. The per-cycle chance of getting pregnant also decreases with age. A woman in her early 40s has about 40% of the monthly fertility of someone in her early 20s. This decline is largely driven by egg quality and quantity, both of which decrease over time.
Sperm quality matters too. Healthy sperm need to be present in sufficient numbers and able to swim forward effectively. The World Health Organization sets a baseline of at least 39 million sperm per ejaculate, with at least 30% of those swimming in a forward direction. Below those thresholds, conception becomes harder.
Assisted Reproduction
When natural conception doesn’t work, several medical options exist. The simplest is intrauterine insemination (IUI), where a doctor places sperm directly into the uterus around the time of ovulation, bypassing the vagina and cervix. This shortens the distance sperm need to travel and is often used when sperm count or motility is low, or for single parents and same-sex couples using donor sperm. The success rate per cycle is modest, averaging around 9%, though it varies widely depending on the underlying cause of difficulty.
In vitro fertilization (IVF) is more involved but more effective. The process starts with fertility medications that stimulate the ovaries to produce multiple eggs instead of the usual one. A doctor then retrieves those eggs using a needle guided by ultrasound. In the lab, the eggs are combined with sperm, and the resulting embryos grow for several days before one or more are transferred into the uterus. The number of embryos transferred depends on factors like age and embryo quality.
Using Donors and Surrogates
Some people need an egg donor, a sperm donor, or someone else to carry the pregnancy. Donor eggs can come from an egg bank (frozen) or from a donor who goes through a fresh IVF cycle. The donated eggs are fertilized with sperm from a partner or a sperm donor, and the resulting embryos are placed in the uterus of the person who will carry the pregnancy.
Surrogacy involves another person carrying the baby. In gestational surrogacy, the surrogate has no genetic connection to the child. Embryos created through IVF using the intended parents’ eggs and sperm (or donor material) are transferred to the surrogate’s uterus. Traditional surrogacy, where the surrogate’s own egg is used and fertilized through insemination, is largely no longer practiced in the U.S. and is illegal in some states. Gestational carriers go through extensive medical and psychological screening and must have a history of uncomplicated pregnancy.
From Fertilization to Pregnancy
Regardless of how the sperm and egg come together, the path from fertilization to confirmed pregnancy follows the same timeline. The fertilized egg divides as it travels to the uterus over about a week. Implantation into the uterine wall happens roughly 6 to 10 days after fertilization. Once implanted, the embryo begins releasing hormones that signal the body to maintain the pregnancy. These are the same hormones that home pregnancy tests detect, which is why most tests are accurate starting around two weeks after conception.
The entire process, from ovulation to a positive pregnancy test, spans about two to three weeks. From there, pregnancy lasts approximately 40 weeks, counted from the first day of the last menstrual period. That means the “first two weeks” of pregnancy actually happen before fertilization occurs, which is why pregnancy math can feel confusing.