Illustration by Marta Pucci
How pregnancy *actually* happens
What is conception?
Top things to know about conception:
Conception is much more complex than just “sperm meets egg”
Only a tiny fraction of the sperm that enter the vagina make it to the fallopian tubes
After fertilization, it takes about 10 days for the egg to travel to the uterus and implant
You can use Clue Conceive to get personalized predictions for when to time sex or insemination
Conception: A complex chain of events
Many of us grew up hearing the basic version: sperm meets egg, egg meets uterus, and bam!—pregnancy begins. While that’s technically true, it leaves out the many finely tuned steps and biological processes that must go just right for conception to occur.
In this article, we break down what happens in the body, step by step, from ovulation through implantation, for people conceiving through penis-vagina sex or other forms of assisted conception (like artificial insemination). While some of these processes are still being studied (especially the ones that happen inside the reproductive tract), we’ll walk you through what researchers currently understand.
What is conception?
Conception refers to the process by which a single sperm fertilizes an egg, and the resulting fertilized egg (zygote) travels to the uterus and implants into the uterine lining. It includes both fertilization and implantation, and both must occur for a pregnancy to officially begin (1).
Timeline of conception
Ovulation: Releasing the egg
Ovulation is the release of a mature egg from the ovary, typically around the middle of a menstrual cycle (1). Finger-like structures called fimbriae carry the egg, guiding it into the fallopian tube, where it may meet sperm (1).
If you're trying to conceive, understanding your ovulation timing is crucial. Clue Conceive can help predict your fertile window with greater accuracy.
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The cervix and vagina prepare for sperm
Leading up to ovulation, estrogen levels rise, and the cervix produces increasing amounts of cervical fluid (1,2). Since cervical fluid follows consistent and recognizable patterns across the menstrual cycle, these changes can offer useful clues about where you are in your cycle and when you might be fertile (3).
This fluid changes in texture throughout the menstrual cycle:
After your period: little or no fluid
Before ovulation: thick, sticky, or creamy fluid (2)
Around ovulation: clear, stretchy, slippery—like raw egg white (2)
After ovulation: thick and sticky (1)
This fertile cervical fluid helps sperm survive and swim toward the uterus. Sperm travel better in this cervical mucus, and it provides a more hospitable environment by temporarily reducing the vagina’s natural acidity (1).
The uterus prepares for implantation
After menstruation, rising estrogen levels help rebuild the endometrium (uterine lining) (2). Once ovulation occurs, progesterone takes over and prompts a series of changes to make the uterus more welcoming for a fertilized egg:
Endometrial cells stop dividing and begin to grow larger
More blood vessels develop to support potential implantation
Small, sugar-rich secretions are released to nourish the incoming embryo (2)
These changes help create the ideal environment for implantation.
What does sperm do after ejaculation?
During unprotected sex, millions of sperm are ejaculated into the vagina. Most never make it far. Only a tiny fraction make it past the cervix and into the uterus (1).
For those that do, it’s the uterus (not the sperm tails!) that does much of the work moving them forward (1). Contractions (partly triggered by the hormone oxytocin released during sex) and tiny hair-like structures called cilia help guide the sperm toward the fallopian tubes (1).
The uterus vs. sperm
Once inside the uterus, sperm trigger an immune response (1,4). White blood cells target and destroy many of them, especially the weaker or abnormal ones (1,4). Some sperm may even travel into the wrong fallopian tube, where no egg awaits (1).
All of this dramatically narrows the field: only a few hundred out of millions of sperm may reach the site of fertilization (1).
Fertilization: When sperm meets egg
If a sperm finds the egg in the fallopian tube, it must undergo two processes:
Capacitation: a biochemical change that preps the sperm to fertilize the egg
Hyperactivation: a change in how the sperm tail moves, giving it more thrust (1)
The egg and surrounding fluid release signals that guide sperm to the right spot, possibly including heat or chemical cues (1).
How sperm enters the egg
To fertilize the egg, a sperm has to overcome two protective layers:
The cumulus oophorus: a cluster of cells that surrounds the egg. Sperm release enzymes that help break down these cells, allowing them to move closer (1).
The zona pellucida: the egg’s outer membrane. One sperm uses more enzymes to bind to and break through this layer (1).
Once one sperm successfully fuses with the egg’s membrane, the egg blocks out all others (1). The sperm’s nucleus merges with the egg’s, forming a zygote with all the genetic material needed to become a human (1).
Conception vs. Implantation: What happens next?
After fertilization, the zygote travels down the fallopian tube in about 10 days. As it divides and develops into a blastocyst, it arrives in the uterus and attempts implantation (1).
Implantation marks the beginning of pregnancy. The uterine lining releases enzymes that allow the blastocyst to attach and embed itself (1).
How is conception detected?
The body starts producing human chorionic gonadotropin (hCG) after implantation (1). This is the hormone detected by pregnancy tests (5).
When things don’t go as planned
Conception is delicate—and doesn’t always succeed.
About 4–6 out of every 10 fertilized eggs fail to implant (2,6). These early losses typically happen before a person even knows they’re pregnant. They're not considered miscarriages by most healthcare professionals, since pregnancy technically begins at implantation.
For people using assisted reproductive technologies (discussed below), each embryo can carry deep emotional significance. When an embryo doesn’t implant, it can feel like a profound loss, even if it’s not clinically defined as a miscarriage.
In rare cases, the fertilized egg implants outside the uterus, usually in the fallopian tube. This is called an ectopic pregnancy, and it requires immediate medical attention (7).
Even though the process is complex, it’s pretty amazing how often our bodies do get it right.
Factors affecting conception
Timing
Your chances of conception are highest during the fertile window—the six days leading up to and including ovulation (3). Tools, such as ovulation predictor kits or cycle tracking with Clue Conceive, can help identify these days.
Health and lifestyle
Smoking, alcohol, diet, and environmental pollutants can all impact fertility for people of all sexes (3).
Medical conditions
Conditions like PCOS, endometriosis, or untreated STIs can make it harder to conceive. If you’ve been trying to get pregnant for a year (or six months if over 35), consider talking to a fertility specialist (8).
Conception with IVF, IUI, and other methods
When conception doesn’t happen the old-fashioned way, there are advanced procedures or assisted reproductive technologies (ART) that can help egg and sperm meet.
Intrauterine insemination (IUI) is a type of artificial insemination where sperm is placed directly into the uterus, bypassing the cervix (1).
In vitro fertilization (IVF) involves retrieving eggs, fertilizing them in a lab, and transferring an embryo to the uterus (1). Fertilization still happens, but outside the body.
Other ARTs may involve donor sperm, donor eggs, or surrogacy (1). Conception still requires fertilization and implantation, even if these steps happen differently.
Signs of early pregnancy
The first signs of pregnancy can vary, but may include a missed period, fatigue, sore breasts, or nausea (1,9)
FAQs
How long is pregnancy from conception?
Pregnancy typically lasts about 38 weeks from conception, or 40 weeks from the first day of your last period (1).
Is conception the day I got pregnant?
Conception is the day the egg is fertilized by a sperm. It usually occurs within 24-48 hours after ovulation, but sperm can survive in the body for up to 6 days, making it difficult to pinpoint the exact day (1).
When should I have sex if I want to get pregnant?
Aim to have sex during your fertile window—the six days leading up to and including ovulation. Studies show that the chance of conception is highest when you have sex (or inseminate) every 1–2 days during the fertile window (3).
