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.
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!
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.
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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.
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.
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.
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.
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.
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.
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.
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.
Every person and every pregnancy is unique. An increase in hormones cause most pregnancy symptoms. Some of the most common are:
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.
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.
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:
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.
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:
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:
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.
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.
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.
Call your provider right away if you have:
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.
Last reviewed by a Cleveland Clinic medical professional on 08/22/2022.
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