First Trimester

What Is the First Trimester of Pregnancy?

Pregnancy has three trimesters. Each trimester is about 13 weeks, or three months long. A full-term pregnancy lasts 40 weeks (between nine and 10 months). Your first trimester starts on the first day of your last menstrual period and goes until the beginning of the 13th week.

Your first trimester is very important. You might not look or feel pregnant at first, but your body is changing. Now is a great time to think about your overall health and lifestyle changes you may need to make during pregnancy.

What are common symptoms of the first trimester?

Every person and every pregnancy is different. But some of the most common symptoms are:

• Sore breasts: Hormones may make your breasts feel tender and swollen. You may notice veins in your breasts and darkened areolas (the skin around your nipple).
• Feeling tired: Your body is hard at work during the first trimester of pregnancy. This may leave you feeling exceptionally tired.
• Nausea: Morning sickness is one of signs of early pregnancy. It can occur at any time, not just the morning.
• Mood swings: The sudden rush of hormones can affect your mood. This can result in getting angry or crying more quickly than usual.
• Needing to pee: Your uterus grows and may begin pressing on your bladder. This causes you to pee more often.
• Acne or other skin changes: Hormones cause your skin to create more oil. This can lead to clogged pores and acne.
• Mild shortness of breath: You may feel out of breath with light physical activity.

The first trimester brings many physical and emotional changes. Your hormone levels rise quickly. In fact, your body produces more estrogen during one pregnancy than it does during your entire life when you’re not pregnant.

Your heart also pumps more blood during pregnancy. This can make your pulse faster and cause you to feel tired more easily. Listen to your body and rest when you feel tired or short of breath.

How does my baby develop in the first trimester?

During the first trimester, many body structures and organ systems develop. Making healthy choices and getting regular prenatal care is important during this time.

Weeks one to four

The structures that will become the amniotic sac, placenta and umbilical cord start to form. A tube that becomes the fetus’ 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

During the sixth week of pregnancy, several major organs begin to form. These include the heart, lungs, ears, arms and legs. Soft tissue slowly turns into bone. The face has a more-defined mouth and nose. Some providers may offer an early ultrasound at this time to check for a heartbeat. By the end of the eighth week, the embryo is called a fetus. It is about 1 inch long, or roughly the size of a raspberry.

Weeks nine to 12

Towards the end of your first trimester, the fetus will have toes, fingers and nails. It will start to move by opening and closing its hands and mouth. Its 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 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.

What tests will I have in the first trimester?

Checkups, screenings and other tests during pregnancy help identify any health issues that may cause problems in your pregnancy. Prenatal care appointments are important because your pregnancy care provider discusses what you can expect, performs checkups and screenings, and answers any questions you may have.

Your first prenatal visit

Once you find out you’re pregnant, contact a pregnancy care provider as soon as you can. If you don’t have one, there are resources that can help you find care, even if you don’t have insurance.

You’ll have either two or three prenatal visits during your first trimester. This can vary depending on your provider or if you have 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.

At your first prenatal visit, your provider will calculate your due date. They’ll check your weight and blood pressure. You can also expect them to perform the following:

• A pelvic exam
• A Pap test (if you’re due for one)
• Tests to check for sexually transmitted infections (STIs)
• Check your pee for bacteria, protein and glucose (sugar)
• Order blood tests to check Rh factor and iron levels
• Check the fetal heart rate if your pregnancy is far enough along to do so

Some providers use transvaginal ultrasound at your first appointment to measure the fetal heart rate and size. This ultrasound also shows if you’re expecting multiples. You’ll have at least one ultrasound in your first trimester. The exact timing varies depending on your provider. If you’re expecting multiples, you may have more 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 more tests.

What should I be doing to stay healthy?

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. 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 lots of water
• Attend your prenatal appointments.

What should I avoid in the first trimester?

After learning you’re pregnant, you may need to make some lifestyle changes. These choices can help support a healthy pregnancy. You should avoid:

• Alcohol
• Cigarettes and tobacco
• Recreational drugs, like cannabis, stimulants and opioids
• Contact sports, like football, soccer, rugby, volleyball, roller derby or activities that put pressure on your abdomen
• Activities that can turn your body into a projectile, such as horseback riding, bicycling, rollerblading, motorcycling, ATV riding or snowmobiling
• Foods like raw fish (sushi), fish high in mercury, uncooked or undercooked meats, lunch meat, and unpasteurized milk, cheese or juice
• Hot tubs and saunas

When should I call my pregnancy care provider?

Call your provider right away if you have:

• Heavy bleeding or unusual vaginal discharge
• Severe cramping in your belly, arms or legs
• Persistent vomiting and/or diarrhea
• Fainting spells or dizziness
• Blurred vision