A pregnancy is when a fetus grows inside your uterus. Pregnancy lasts about 40 weeks and is split into three trimesters that each last around 13 weeks. Getting prenatal care is essential to a healthy pregnancy.

Growth and size of a fetus from six weeks to full term
A fetus grows in the uterus during pregnancy. A full-term pregnancy lasts between 39 and 40 weeks and has three trimesters.

What is pregnancy?

Pregnancy refers to a time when you have a fetus (or more than one) developing in your uterus. It most often happens after sexual intercourse, but it can also happen through assisted reproductive technologies (ARTs). An at-home pregnancy test is the most common way to confirm pregnancy, but you can also confirm you’re pregnant with a blood test. Some of the first signs of pregnancy include a missed period, nausea and fatigue.

Most pregnancies can end in a live birth — either a vaginal delivery or a C-section. Some pregnancies end in miscarriage, abortion or stillbirth.


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How does pregnancy happen?

Pregnancy is a complex process consisting of several steps, but it all starts with an egg and sperm:

  • Eggs live in ovaries. Each menstrual cycle, your ovary releases an egg. The egg slowly travels through your fallopian tube where it waits for sperm. This process, which is called ovulation, takes about 12 to 24 hours.
  • Sperm comes from testicles. When a person ejaculates, they release millions of sperm from their penis. During sexual intercourse, sperm travels through your vagina and up to your fallopian tubes.

For a pregnancy to happen, a sperm and egg need to meet and come together (conception). The sperm fertilizes the egg in a process called fertilization. The now-fertilized egg travels down your fallopian tube where it divides into more and more cells, forming a collection of cells called a blastocyst. After about three days, the blastocyst gets to your uterus. At this point, the blastocyst can implant into your uterus (a step called implantation), where it’s now an embryo. The placenta begins to form after implantation. The embryo changes once more to become a fetus, which is the term your provider may use until your baby’s birth.

Once implantation happens, your body releases pregnancy hormones to prevent you from getting your period. These same hormones also help with fetal development.

Assisted reproductive technology (ART)

The process occurs the same way with ART, but some of the steps happen outside of your body. The stars of the show are still the sperm and the egg, but instead of conception occurring naturally or by chance, your healthcare provider helps by making it slightly easier for them to come together. Here’s how some of the more common ARTs work:

  • Intrauterine insemination (IUI): Your healthcare provider places sperm directly into your uterus using a catheter. IUI occurs at the time of ovulation. Some people take fertility medication to increase their chances of ovulation.
  • In vitro fertilization (IVF): Your healthcare provider combines the egg and sperm in a lab to create an embryo. Then, they place the embryo inside your uterus. Your healthcare provider may give you several different types of medication during an IVF cycle. This helps them control the process and improve your chances of a successful pregnancy.

How many weeks is pregnancy?

Pregnancy is 40 weeks or 280 days. But it’s slightly more complex and harder to understand than that, mostly due to how you calculate a pregnancy.

Your healthcare provider typically determines how far along in pregnancy you are based on the date of your last menstrual period (LMP). This gets confusing because ovulation doesn’t happen until about two weeks into your cycle. So, by the time you take a pregnancy test two weeks after ovulation, you’re already four weeks pregnant.

Try not to let the math of pregnancy confuse you. Your pregnancy care provider can help explain it to you and tell you your due date based on a pregnancy ultrasound.

If you get pregnant through IVF, the weeks in pregnancy are still the same, but your provider may calculate it differently. This is mainly because they transfer the embryo into your uterus and you’re removing some of the steps that take about two weeks in a natural conception.

How do you calculate your due date?

There are also many due date calculators available online to help you determine your due date. Another way to estimate your date is to:

  • Write down the date of your LMP.
  • Add seven days.
  • Count back three months.
  • Add a year.

Even if you calculate your due date, your pregnancy care provider will confirm your due date at your appointment (or offer a new due date based on ultrasound results). Going to your prenatal visits also helps your provider monitor the accuracy of your due date.

Keep in mind that the chances of giving birth on your estimated due date are only about 5%.

What does gestational age mean?

Gestational age basically means how far along the pregnancy is. Gestational age is also confusing because it goes by your LMP, which means it’s counting the days before you’re pregnant. It essentially accounts for the time before ovulation when your body is preparing for a pregnancy.

Gestational age is a combination of weeks and days. For example, 22 weeks and 3 days pregnant. You may see this same figure written as 22 3/7 or referred to as 22 weeks gestation.

Gestational age describes the pregnancy, not the fetus. The fetal age is typically not a measurement your provider will use. This is mainly because you’d need to know exactly when conception happens, which is very hard to do.


What are the trimesters of pregnancy?

There are three trimesters of pregnancy: first, second and third. They each last about three months or 13 weeks.

What can I expect in each trimester?

Each trimester is unique and comes with different symptoms. Your body (and the fetus) will change and grow a lot each trimester. For example, your uterus begins as the size of a lemon and grows to the size of a watermelon!

First trimester

The first trimester lasts until you’re 13 weeks pregnant. Contact a pregnancy care provider as soon as you know you’re pregnant.

Here are just a few of the many things you can expect in the first trimester:

Your pregnancy care provider will recommend taking a prenatal vitamin. They’ll also give you a list of things to avoid during your pregnancy. Some of those things include:

  • Beverages containing alcohol.
  • Tobacco products.
  • Recreational drugs, including marijuana. Medical marijuana should be discussed with your pregnancy care provider.
  • Certain foods like unpasteurized dairy, raw fish or fish high in mercury.
Second trimester

The second trimester consists of weeks 13 to 28.

People who feel nauseous and tired in the first trimester often feel “better” in the second trimester. You can also bring some symptoms from the first trimester with you to the second trimester. Most people begin feeling the fetus move in the second trimester. By about 17 weeks of pregnancy, most people have gained at least 10 pounds.

Some of the second-trimester symptoms are:

  • Body and muscle aches.
  • Weight gain.
  • Darkening of your areolas (the colored skin around your nipple).
  • Developing a linea nigra (dark line that runs from your pubic bone to your belly button).
  • Patches of dark skin.
Third trimester

You’re in the home stretch! The third trimester lasts from week 29 to 40. You can expect aches and pains from the fetus (and your belly) continuing to grow. You can expect to gain a few more pounds as the fetus puts on the extra fat it needs to stay warm at birth.

Some of the things you can expect in the third trimester are:

  • Becoming short of breath more easily.
  • Lower back pain, which is common due to a natural arching of the back when your growing belly pulls your weight forward.
  • Needing to pee more frequently.
  • Trouble getting comfortable for sleep.
  • Your breasts leaking colostrum, the first form of breast milk.
  • The fetus dropping into your pelvis to prepare for birth. This is called lightening.
  • Signs of labor like contractions or losing your mucus plug.

What does full-term pregnancy mean?

Healthcare providers also talk about pregnancy in terms. A full-term pregnancy is a pregnancy that lasts between 39 weeks, 0 days and 40 weeks 6 days. Babies who are born full term have the best chances of being healthy. There are other terms your provider may use, such as:

  • Early term: Your baby is born between 37 weeks, 0 days and 38 weeks, 6 days.
  • Late term: Your baby is born between 41 weeks, 0 days and 41 weeks, 6 days.
  • Post term: Your baby is born after 42 weeks.

What kind of care do I need during pregnancy?

Prenatal care is critically important for your health and the fetus’s health. Regular prenatal appointments help your provider monitor your health and detect pregnancy complications. They also use the appointments to check that the fetus is growing as expected.

Some of the things your pregnancy care provider will do are:

Prenatal appointment schedule

The exact number of appointments you have during pregnancy varies. People with medical conditions or complications may need to see their provider more often. If your pregnancy is healthy and there are no concerns, your pregnancy care provider will typically see you:

  • Every four weeks up until the 28th week of pregnancy.
  • Every two weeks from week 28 to week 36.
  • Every week from week 36 until you deliver.

How much weight should I gain during pregnancy?

There isn’t an exact number because everyone’s body is different. Most healthcare providers prefer a pregnant person’s weight gain to fall somewhere between 25 to 35 pounds.

A note from Cleveland Clinic

Pregnancy can be an overwhelming time in your life. There are feelings of joy, fear, anticipation and many other emotions. Your body will go through so many changes, and it can be hard to know what’s expected and what’s not. It’s natural to have tons of questions about what you can expect and what you should do to have a healthy pregnancy. The best thing you can do in pregnancy is have regular visits with your pregnancy care provider. They’re your best resource as you navigate this life-changing event. It’s always OK to ask them questions so they can put your mind at ease.

Medically Reviewed

Last reviewed on 04/11/2024.

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