How long after sex does conception occur?

We know that conception occurs when one sperm joins with one egg, but how long after sex does that take?

When it comes to making a baby, the odds are stacked against any single sperm cell ever becoming a zygote through contact and penetration of a female egg. That's if a man has perfectly healthy sperm -- and if he has enough of them.
Let's look at the numbers: In the time it takes the human heart to produce one beat, a man's body produces 1,000 sperm cells [source: National Geographic]. On average, a man's ejaculate (about 2.75 milliliters of it) contains over a quarter-billion sperm [source: Lindemann]. Anything less than 20 million sperm per milliliter of semen is considered low, while 39 million or more per milliliter is considered optimal for fertilization [source: Mayo Clinic].
One study showed that more intimate sex causes men to produce higher quantities of sperm that are of higher quality [source: Campbell]. But many other factors can affect the number and health of sperm cells, including environmental causes, lifestyle choices and medical issues.
Smoking, heavy drinking, drug use (including prescription drug use) or lack of exercise all contribute to diminished sperm count or poor sperm health. By maintaining a healthy lifestyle -- exercising, taking a daily multivitamin, eating nutritious meals with fruits and veggies -- you'll improve your odds of developing healthy sperm that are optimal for fertilization. Regardless of health or lifestyle, after age 50 there's generally a decline in sperm motility and quantity. And too much time in a hot bath or sauna can even affect sperm health, as the testes maintain a lower temperature that's preferable for the health of the sperm within.
Let's say that a man has perfectly healthy sperm cells. While a quarter-billion or so start off in search of the egg, fewer than 100 may ever get close to it.
But how long does it actually take for a sperm to penetrate an egg and form a zygote? What's the shortest time it would take? What's the longest? Keep your pants on -- we'll begin to uncover those answers in the next section.

What affects the success rate of conception?

While there's disagreement about whether or not a man should refrain from sex and ejaculation for a period of time before his partner ovulates, recent research indicates a man's sperm are healthiest when he has had daily sex in the week leading up to attempted conception [source: von Radowitz].
It seems the constant renewal of sperm prevents the time-related DNA damage that can occur in sperm that spend too much time in the testes. Dr. David Greening from the Sydney IVF clinic conducted a study involving men who had a higher-than-usual rate of DNA damage in their sperm cells. His team found that the percentage of sperm with damaged DNA decreased following periods of daily sex. Though the actual sperm count was cut by half, the sperm that were present were stronger, healthier and more mobile. However, the sperm count dropped to less effective levels if couples had sex three times a day or more [source: von Radowitz].
The odds of successful fertilization increase if at least 40 percent of the sperm are moving [source: Mayo Clinic]. Not all sperm cells have similar structures -- some have misshapen heads or damaged tails. In fact, many of the sperm are flawed in some way. Luckily, fertilization is probable if only 4 percent or more of the sperm have normal forms -- an oval head and a long tail to help it swim its way to conception [source: Mayo Clinic].
For women, any abnormality in the reproductive system can cause major problems when it comes time to conceive. Damaged fallopian tubes or uterine issues can prevent conception or a successful pregnancy.
In addition to receiving regular medical checkups, women can improve the odds of conception by staying healthy, maintaining a proper weight, staying away from tobacco, caffeine, alcohol and drug use, and protecting themselves against sexually transmitted diseases.
To increase the odds of pregnancy, have sex at least every other day around the time of ovulation [source: Mayo Clinic]. Also, talk to your doctor to find out which lubricants may be adversely affecting the mobility -- and eventual success -- of the sperm.
Assuming both parties are healthy, how long does it take for fertilization to occur once sperm cells have entered the vagina? We'll find out in the next section.

A fast sperm cell could have reached its goal while you're still curled up in bed together.

From Sex to Conception

There are close to equal numbers of sperm that contain either a male chromosome or a female chromosome. This determines the gender of the baby at the very point of egg fertilization. It will also play a role in the race to reach the egg. While male sperm aren't as hardy, they have a speed advantage. Though slower than their male counterparts, the advantage in longevity goes to female sperm.
Sperm ejaculated outside of a female's body have a lifespan that tops out at a few hours and may be as brief as a few minutes [source: Harms]. But if they survive long enough to reach the fallopian tube, sperm cells can relax and enjoy a safe environment that's conducive to their continued survival. Some actually bind to the fallopian tube itself, as a means of receiving direct nourishment [source: National Geographic].
When an egg enters the fallopian tube from the other end, it produces a scent that drives the sperm cells crazy. They become very excited and begin a process called capacitation, during which they shed certain proteins and become very excited. Both the shedding of proteins and the boost in mobility aid the sperm cell in its quest to penetrate the egg.
When a sperm cell finally makes contact with the egg, the head of the sperm releases enzymes that help it penetrate the egg's exterior. Once a sperm cell does, the two cells -- the sperm and the egg -- form a single-celled unit called a zygote, and the change prevents any other sperm cells from getting through.
The fastest sperm can get to a fallopian tube is about 30 minutes, meaning that the quickest conception could occur following sex is in the half-hour range [source: WebMD]. This means that, following sex, the egg could be fertilized before you've gotten up to get a drink of water.
Conception can occur as many as five days after sex or possibly longer, as strong, healthy sperm can survive for about that many days (and perhaps even longer) in the supportive environment of the fallopian tubes as they wait for an egg to be released, if one isn't already present [source: Harms].
You can click on the next page for lots more information on fertility and conception in far less time than it takes to create a zygote.