First Trimester

The first trimester lasts until the end of the 13th week of pregnancy. You may begin feeling signs of pregnancy like nausea and tender breasts. Fetal development starts when the egg is fertilized. By the end of the 13th week, all of its organs and body systems are developing. Find a pregnancy care provider as soon you know you’re pregnant.

What is the first trimester of pregnancy?

Pregnancy has three trimesters or stages. Each trimester is about 13 weeks or three months long. A full-term pregnancy lasts 40 weeks or between nine and 10 months. Your healthcare provider will talk to you about fetal development in terms of weeks. Your first trimester of pregnancy lasts until the 13th week of pregnancy.

It may seem strange, but your pregnancy actually begins on the first day of your last menstrual period. This is called the gestational age of the pregnancy. A pregnancy care provider calculates your due date by adding 40 weeks to the first day of your last menstrual period. So this means, by the time you know you’re pregnant, you’re already about four weeks along. This can be very confusing!

Understanding weeks of pregnancy

The first two weeks of pregnancy are part of your normal menstrual cycle — the first week is your period and the second week is ovulation. Once you ovulate, your egg travels through your fallopian tube to your uterus. If it meets up with sperm, they combine and conception occurs (fertilization).

During the third week of pregnancy, the fertilized egg travels to your uterus. On its way down to your uterus, it divides into more cells. Once it reaches your uterus, it implants into your uterine lining. This triggers your body to recognize that you’re pregnant and a series of changes begin to happen. Most people miss their period and then get a positive pregnancy test.


Cleveland Clinic is a non-profit academic medical center. Advertising on our site helps support our mission. We do not endorse non-Cleveland Clinic products or services. Policy

How long is the first trimester?

The first trimester begins before you’re pregnant. It starts on the first day of your last menstrual period and goes until the 13th week of pregnancy.

What can I expect in my first trimester?

Your first trimester of pregnancy is full of many physical and emotional changes. It can be a very overwhelming time, and your mind may be racing with questions. Plus, your hormones are in overdrive. In fact, your body produces more estrogen during one pregnancy than it does during your entire life when you’re not pregnant. This surge in hormones can cause some unpleasant pregnancy symptoms. You may find yourself feeling moody, bloated and tired. While you may not see a prominent baby bump yet, your uterus is expanding and your blood volume is increasing.

It’s OK to feel both excited and nervous. Talking to your friends, partner or a healthcare provider may help you feel better as you navigate your pregnancy journey.


What should I do in my first trimester?

Your first trimester is very important. You might not look or feel pregnant, but lots of changes are happening.

If you don’t have a healthcare provider or a pregnancy care provider, you should find one as soon as possible. Getting early pregnancy care can help you avoid any potential complications. Make a list of questions or concerns you have so you’re ready for your first appointment. Check with your health insurance about pregnancy coverage so you know what to expect and where you can get care. If you don’t have health insurance, there are programs and agencies to help you get prenatal care.

There are different types of pregnancy care providers that take care of you during pregnancy, labor, delivery and postpartum. These include obstetricians, midwives, and sometimes, primary care physicians. In addition to selecting a pregnancy care provider, you may also consider places to deliver your baby. While most people choose to give birth in a hospital, some people prefer birthing centers or home births.

Now is a great time to think about your overall health and what lifestyle changes you may need to make now that you’re pregnant. For example, think about how pregnancy affects your work, finances, habits and daily activities.

How does the fetus develop in the first trimester of pregnancy?

Several developments occur in the first trimester. Although you can’t see it happening, there’s a lot going on inside your body after sperm fertilizes an egg.

Weeks one to four of pregnancy

During the first month of pregnancy, several important structures form. These structures are a tiny clump of cells, but will grow to become the amniotic sac, placenta and umbilical cord. A tube that becomes the fetus’s brain and spinal cord forms, as well as its circulatory system. A face, circles for eyes and the beginning of a mouth take shape.

The embryo is about a quarter-inch inch long — smaller than a grain of rice.

Weeks five to eight of pregnancy

Several major organs begin to develop during the sixth week of pregnancy including the fetal lungs, heart, ears, arms and legs. Bones begin to replace tissue. Its head is large in proportion to the rest of its body, but it look more human now. The fetus has a distinct mouth, nose and face. Some providers do an early ultrasound to confirm a heartbeat during this time.

By the end of the eighth week of pregnancy, the embryo becomes a fetus. It’s about 1 inch long or the size of a raspberry.

Weeks nine to 12 of pregnancy

Towards the end of your first trimester, the fetus will have toes, fingers and nails. It will start to move by opening and closings its hands and mouth. The fetus’s urinary and digestive systems are also fully functioning. At around 12 weeks of pregnancy, your provider can listen to the fetal heart using a Doppler ultrasound. It also has either a vagina or a penis at this point (though your provider can’t see it on an ultrasound).

By the end of the 12th week of pregnancy, the fetus is between 3 and 4 inches long — about the size of a plum. It weighs about 1 ounce.


Why is the first trimester of pregnancy so critical?

The first trimester is so important because most of the fetus’s major organs and body systems are developing. Toxins, harmful substances and infection can severely damage a fetus’s growth and development during this time. It could increase your baby’s risk of being born with a congenital disorder.

What are the most common symptoms during the first trimester?

Every person and every pregnancy is unique. An increase in hormones cause most pregnancy symptoms. Some of the most common are:

  • Sore breasts: Hormones may make your breasts feel tender and large. It’s common to need bigger bras before the end of your first trimester. The veins in your breasts may become noticeable because they’re carrying more blood. Other changes to your breasts may include darkened areolas or changes to your nipples.
  • Nausea: Morning sickness is one of the telltale signs of early pregnancy. Despite its name, it can last all day and all night. Try eating smaller meals or bland, low-fat foods. Some people find relief by eating foods containing ginger.
  • Mood swings: The sudden rush of hormones may put you on a rollercoaster of emotions. You may alternate between feeling anxious or scared to excited or weepy within a span of 30 minutes. It may be helpful to talk through your feelings with a friend or your partner.
  • Feeling tired: Your body is hard at work during the first trimester of pregnancy. This may leave you feeling exceptionally tired. Be sure to get plenty of rest. Most people get some energy back in the second trimester.
  • Needing to pee: Your uterus begins to grow to support the pregnancy. It may begin pressing on your bladder, causing you to need to pee more often.
  • Acne or other skin changes: Hormones cause your skin to create more oil during pregnancy. This can lead to clogged pores and acne in some people. There are other skin conditions that appear during pregnancy, but most appear in the second or third trimesters.
  • Mild shortness of breath: You may feel short of breath with light physical activity.

Your heart is pumping more blood during pregnancy. This means your pulse may be quicker and you may find yourself losing energy more easily. Be mindful of how much demand pregnancy puts on your body and take rests when you feel tired or out of breath.

What tests will I have in the first trimester of pregnancy?

Checkups, screenings and other tests during pregnancy help keep you and the fetus healthy. Care during pregnancy is commonly referred to as prenatal care. Prenatal care appointments are important because your pregnancy care provider discusses what you can expect during pregnancy and delivery, performs checkups and screenings and answers any questions you have.

Your first prenatal visit

You’ll have between two and three prenatal visits during your first trimester. This can vary depending on your provider or if you’re a high-risk pregnancy. You can expect to discuss your personal medical history, gynecological and obstetrical history (prior pregnancies and births), as well any family medical history that may affect your pregnancy. This visit is very thorough to make sure you and the growing fetus are healthy.

At your first prenatal visit your provider will calculate your due date. You can also expect them to perform the following:

  • A physical exam, including checking your weight and blood pressure.
  • A pelvic exam.
  • A Pap test (if you’re due for one).
  • Tests to check for certain sexually transmitted infections (STIs).
  • Check your pee for bacteria, protein and glucose (sugar).
  • Order blood tests to check hormone levels, Rh factor, iron levels and certain diseases.
  • Check the fetal heart rate.

Some providers use transvaginal ultrasound at your first appointment to confirm pregnancy and measure the fetal heart rate and size. This ultrasound also shows if you’re expecting multiples. A transvaginal ultrasound involves your provider placing a wand inside your vagina. Most pregnant people are offered at least one ultrasound in their first trimester, but the exact timing varies depending on your provider. If you’re expecting multiples, you may be offered additional ultrasounds in your first trimester.

Your provider may suggest other screening tests during pregnancy. Screening tests identify if you or the fetus are at risk for certain health conditions. Based on the results of your screening, you may need diagnostic tests. Diagnostic tests confirm or rule out health problems. In the first trimester, your provider may suggest a screening to detect a higher risk of chromosomal disorders like Down syndrome. Talk to your provider about the screenings they recommend.

What should I not do in the first trimester of pregnancy?

Once you find out you’re pregnant, it’s normal to have to make some lifestyle changes. These changes help ensure that everyone is healthy. You should avoid the following things during your first trimester of pregnancy:

  • Alcohol.
  • Cigarettes and tobacco.
  • Recreational drugs like opioids.
  • Contact sports like football or activities that put pressure on your abdomen.
  • Foods like raw fish (sushi), fish high in mercury, uncooked or undercooked meats, lunchmeat and unpasteurized milk, cheese or juice.
  • Hot tubs and saunas.

How do I take care of myself in the first trimester of pregnancy?

Staying healthy is important throughout all three trimesters of pregnancy. Here are some helpful tips on staying healthy during the first 13 weeks of pregnancy:

  • Stay active as much as you can. Listen to your body and stop for rest if you feel any discomfort while exercising. You may need to modify your exercise routine during pregnancy.
  • Take a prenatal vitamin containing folic acid.
  • Eat a variety of healthy foods including fruits, vegetables, meat, eggs and whole grains.
  • Get plenty of rest.
  • Drink plenty of water.
  • Attend all of your prenatal appointments.

Is it normal to bleed during the first trimester of pregnancy?

Light bleeding or spotting during pregnancy is usually OK in the first few weeks of pregnancy. Some people experience implantation bleeding (when a fertilized egg burrows unto your uterine lining). Call your pregnancy care provider if you’re bleeding heavily or the bleeding lasts more than one day.

What prenatal vitamin should I take?

The vitamins and minerals in your food (or in prenatal vitamins) help support the fetus as it grows and develops. Most providers recommend taking a prenatal vitamin as soon as you begin trying to get pregnant. Vitamins containing folic acid, iron and calcium help support a healthy pregnancy. Talk to your provider if you’re unsure about which prenatal vitamin to take.

Can I drink caffeine during pregnancy?

Most healthcare providers recommend limiting caffeine consumption to under 200 milligrams per day during pregnancy. That’s about 12 ounces of coffee or three cans of Mtn Dew®. This is because a fetus can’t metabolize caffeine so it can build up in their body and cause complications.

When should I call my pregnancy care provider during the first trimester?

Call your provider right away if you have:

  • A fever higher than 100.4 degrees Fahrenheit.
  • Heavy bleeding or unusual vaginal discharge.
  • Severe cramping in your belly, arms or legs or abdominal pain.
  • Persistent vomiting and/or diarrhea.
  • Fainting spells or dizziness.
  • Swelling in your hands, fingers or face.
  • Blurred vision or spots before your eyes.

A note from Cleveland Clinic

Pregnancy is an exciting, and sometimes scary, time in your life. You may feel overwhelmed with information and have lots of questions. During the first trimester of pregnancy, your body is growing and changing rapidly. The fetus is growing and developing, too. In fact, by the end of the first trimester, the fetus is the size of a lemon. You may begin having symptoms of pregnancy like nausea, sore breasts or needing to pee more often. Schedule an appointment with a pregnancy care provider as soon as you know you’re pregnant. Regular prenatal care is important so you and the fetus stay healthy and strong during pregnancy.

Medically Reviewed

Last reviewed on 08/22/2022.

Learn more about our editorial process.

Appointments 216.444.6601