Pregnancy begins when a sperm fertilizes an egg and the resulting embryo attaches to the lining of the uterus. That sequence sounds simple, but it depends on precise hormonal timing, a narrow window of fertility each month, and a series of biological steps that all need to go right. Here’s how each stage works.
Ovulation: Releasing the Egg
Everything starts with ovulation. Once per menstrual cycle, one of the ovaries releases a single mature egg. This doesn’t happen randomly. In the days leading up to ovulation, a developing follicle in the ovary produces rising levels of estrogen. When estrogen stays high enough for roughly 50 hours, it triggers a surge of luteinizing hormone (LH) from the brain. Ovulation follows about 10 to 12 hours after that LH surge peaks, or roughly 34 to 36 hours after the surge first begins. This is why ovulation predictor kits measure LH in urine: the surge is the most reliable signal that an egg is about to be released.
The egg, once released, is swept into the fallopian tube. It remains viable for fertilization for less than 24 hours. If sperm aren’t already present or don’t arrive in time, the egg breaks down and pregnancy can’t occur that cycle.
The Fertile Window
Because the egg lives less than a day but sperm can survive inside the reproductive tract for several days, the fertile window is wider than most people expect. The highest chance of pregnancy comes from intercourse in the two days before ovulation, with the single best day being the day before ovulation. Conception probability actually starts to decline on ovulation day itself.
One practical finding: in studies of fertile couples, pregnancy rates were similar whether couples had intercourse daily, every other day, or even every three days during the fertile window. The only timing that clearly lowered the odds was having intercourse just once during the entire window.
Cervical Mucus as a Fertility Signal
Your body gives a visible cue about fertility. As estrogen rises in the days before ovulation, the cervix produces increasing amounts of mucus that becomes clear, stretchy (often stretching over an inch), and slippery. This “peak type” mucus appears for an average of about 6 days per cycle and marks the most fertile days. After ovulation, progesterone rises sharply and mucus production drops off, becoming thicker and less hospitable to sperm. Tracking these changes is one of the simplest ways to estimate your fertile window without any tests.
How Fertilization Happens
Fertilization takes place in the outer third of the fallopian tube, the section closest to the ovary. To get there, sperm undergo a process called capacitation, a series of changes that increase their motility and prepare them to penetrate the egg’s outer layers.
The egg is surrounded by two protective barriers. The outer layer is a cloud of cells embedded in a gel-like substance. Sperm release an enzyme that digests through this layer. Beneath it sits a thicker protein shell. A second enzyme on the sperm’s surface dissolves a path through this shell, allowing the sperm to fuse with the egg.
The moment one sperm enters, the egg triggers an immediate chemical reaction. A burst of calcium causes the egg to release substances that harden the outer shell and destroy the receptor proteins that other sperm would need to attach. This prevents more than one sperm from entering, which would create a fatal chromosomal imbalance. Within hours, the genetic material from the sperm and egg combine to form a single cell with a complete set of chromosomes. That cell is now a fertilized egg, or zygote.
From Fertilization to Implantation
The fertilized egg doesn’t implant right away. It spends the next several days dividing as it travels down the fallopian tube toward the uterus. By about day five, it has become a ball of roughly 100 cells called a blastocyst.
Meanwhile, the uterus has been preparing. After ovulation, progesterone transforms the uterine lining from a thin, relatively flat surface into a thick, nutrient-rich environment. By about seven days after ovulation, the lining reaches peak receptivity. Small projections develop on the surface cells, creating attachment points for the embryo. This receptive state lasts only one to two days, forming a narrow “implantation window.”
Implantation typically occurs around nine days after ovulation, though it can happen anywhere from six to twelve days after. The blastocyst burrows into the uterine lining, establishing a connection with the mother’s blood supply. This is the moment that pregnancy truly begins in a biological sense, because only after implantation does the embryo start producing hCG, the hormone that pregnancy tests detect.
When a Pregnancy Test Can Detect It
After implantation, hCG first becomes detectable in blood and urine between 6 and 14 days after fertilization. Because implantation timing varies, this means a home pregnancy test can sometimes pick up hCG as early as 10 days after ovulation, but many pregnancies won’t produce a detectable level until closer to the expected period date. Testing too early is the most common reason for a false negative. If you get a negative result but your period doesn’t come, testing again a few days later often gives a more reliable answer.
How Age Affects the Odds
Not every cycle results in pregnancy, even when timing is perfect. In a large North American study of couples actively trying to conceive, about 79% of women aged 25 to 27 were pregnant within 12 cycles. That proportion held relatively steady through the early 30s: 77% for ages 28 to 30, and 75% for ages 34 to 36.
The sharper decline begins in the late 30s. Women aged 37 to 39 had a 67% cumulative pregnancy rate at 12 cycles. For women 40 to 45, it dropped to about 56%. On a per-cycle basis, women aged 40 to 45 were roughly 60% less likely to conceive in any given cycle compared to women aged 21 to 24. The decline is approximately linear with age, meaning there’s no single “cliff” but rather a steady, gradual decrease in the probability of conception each month.
These numbers reflect natural conception without fertility treatment, and they include all the biological factors that shift with age: fewer remaining eggs, lower egg quality, and subtle changes in hormonal signaling. For couples who haven’t conceived after 12 months of well-timed attempts (or six months if the woman is over 35), a fertility evaluation can help identify whether something specific is slowing things down.