Conception is the time when sperm travels up through the vagina, into the uterus, and fertilizes an egg found in the fallopian tube.

Conception — and ultimately, pregnancy — can involve a surprisingly complicated series of steps. Everything must fall into place for a pregnancy to be carried to term.

Let’s take a closer look at what conception is, when and how it occurs, and potential complications that could affect a pregnancy at each stage.

Conception occurs during the part of a woman’s menstrual cycle called ovulation. Doctors consider day 1 of a menstrual cycle the first day of a woman’s period.

Ovulation usually occurs around the midpoint of a woman’s menstrual cycle. This would fall around day 14 in a 28-day cycle, but it’s important to remember that even normal cycle lengths can vary.

During ovulation, one of the ovaries releases an egg, which then travels down one of the fallopian tubes. If there’s sperm present in a woman’s fallopian tube when this happens, the sperm could fertilize the egg.

Usually, an egg has about 12 to 24 hours where it can be fertilized by sperm. However, sperm can live for several days in a woman’s body.

Therefore, when the ovary releases the egg, sperm that are already present from intercourse a few days before could fertilize it. Or, if a woman has sex during the time the egg has been released, the sperm could fertilize the just-released egg.

Conception comes down to timing, the health of a woman’s reproductive tract, and the quality of a man’s sperm.

Most doctors usually recommend having unprotected sex starting about three to six days before you ovulate, as well as the day you ovulate if you wish to become pregnant. This increases the chances that sperm will be present in the fallopian tube to fertilize the egg once it’s released.

Conception-related concerns

Conception requires several steps to come together. First, a woman must release a healthy egg. Some women have medical conditions that prevent them from ovulating altogether.

A woman must also release an egg healthy enough for fertilization. A woman is born with the number of eggs she will have throughout her lifetime. As she gets older, the quality of her eggs diminishes.

This is most true after age 35, according to the Royal College of Obstetricians and Gynaecologists.

High-quality sperm are also required to reach and fertilize the egg. While only one sperm is needed, the sperm must travel past the cervix and uterus into the fallopian tubes to fertilize the egg.

If a man’s sperm aren’t motile enough and can’t travel that far, conception can’t occur.

A woman’s cervix must also be receptive enough for the sperm to survive there. Some conditions cause the sperm to die before they can swim to the fallopian tubes.

Some women may benefit from assisted reproductive technologies like intrauterine insemination or in vitro fertilization if there are issues preventing healthy sperm from meeting a healthy egg naturally.

Sperm usually fertilizes the egg in the fallopian tube. This is a pathway from the ovary to a woman’s uterus.

An egg takes about 30 hours to travel from the ovary down the fallopian tube, according to the University of California San Francisco.

As the egg travels down the fallopian tube, it lodges in a specific portion called the ampullar-isthmic junction. It’s here that sperm usually fertilize the egg.

If the egg is fertilized, it will usually rapidly travel into the uterus and implant. Doctors call the fertilized egg an embryo.

Implantation-related concerns

Unfortunately, just because an egg is fertilized, it doesn’t mean that a pregnancy will occur.

It’s possible to have damaged fallopian tubes due to a history of pelvic infections or other disorders. As a result, the embryo could implant in the fallopian tube (improper location), which would cause a condition called an ectopic pregnancy. This can be a medical emergency because the pregnancy cannot continue and can cause fallopian tube rupture.

For other women, the blastocyst of fertilized cells may not implant at all, even if it reaches the uterus.

In some cases, a woman’s uterine lining isn’t thick enough for implantation. In other cases, the egg, sperm, or portion of the embryo may not be high quality enough to successfully implant.

After a sperm fertilizes an egg, cells in the embryo start to rapidly divide. After about seven days, the embryo is a mass of multiplied cells known as a blastocyst. This blastocyst will then ideally implant in the uterus.

As the egg travels through the fallopian tube before implantation, though, the levels of the hormone progesterone begin rising. The increased progesterone causes the uterine lining to thicken.

Ideally, once the fertilized egg arrives in the uterus as a blastocyst embryo, the lining will be thick enough so it can implant.

Altogether, from the point of ovulation to implantation, this process can take about one to two weeks. If you have a 28-day cycle, this indeed takes you to day 28 — usually the day when you would start your period.

It’s at this point that most women can consider taking an at-home pregnancy test to see if they’re pregnant.

At-home pregnancy tests (urine tests) work by reacting with a hormone present in your urine known as human chorionic gonadotropin (hCG). Also known as the “pregnancy hormone,” hCG increases as your pregnancy progresses.

Keep a few things in mind as you take an at-home pregnancy test:

First, the tests vary in their sensitivity. Some may require higher amounts of hCG to yield a positive.

Second, women produce hCG at varying rates when they get pregnant. Sometimes a pregnancy test can yield a positive one day after a missed period, while others can take a week after a missed period to show a positive.

Post-conception-related concerns

Conception doesn’t always mean that a pregnancy will occur and be carried to full term.

Sometimes, a woman may have a miscarriage in the pregnancy before the embryo implants or shortly after. She may have miscarriage-related bleeding around the time she’s expecting her period and never realize conception took place.

Several other conditions can occur, such as a blighted ovum. This is when a fertilized egg implants in the uterus, but develops no further. On an ultrasound, a doctor may observe an empty gestational sac.

According to the American College of Obstetricians and Gynecologists, an estimated 50 percent of all early miscarriages are due to chromosome abnormalities. If the sperm and egg don’t have 23 chromosomes each, the embryo can’t develop as expected.

Some women may experience pregnancy loss with no known cause. This is understandably difficult for all involved. However, this doesn’t mean a woman can’t get pregnant again in the future.

In vitro fertilization (IVF) is an assisted reproductive technology that involves using sperm to fertilize an egg in a laboratory setting. This creates an embryo.

A doctor then places the embryo into the uterus, where it will ideally implant and pregnancy will occur.

In the case of natural pregnancy, doctors often use the estimated date of conception to estimate a baby’s due date. This wouldn’t be accurate for a person going through IVF, because conception (sperm fertilizing egg) technically occurs in a laboratory.

Doctors may use different methods to estimate due date for an IVF pregnancy. Often, they use the date the eggs were fertilized (an embryo was formed) or when the embryos were transferred.

In either natural or assisted conception, it’s important to remember that while a due date can give you a date to plan for, few women deliver on their due date.

Factors such as how large a baby is measuring and seems to be developing can be better ways to guess a baby’s gestational age as a pregnancy progresses.

While conception technically refers to a single sperm fertilizing an egg, there’s much more to getting pregnant than conceiving.

If you have questions about the steps of conception or your ability to get pregnant, talk to your doctor.

If you don’t get pregnant after one year of unprotected sex (or six months if you’re over the age of 35), ask about potential causes and treatments that can enhance your chances of conception and pregnancy.