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When you're pregnant, the first trimester is an exciting and anxious time. Hormones can cause significant emotional and physical changes. You're given many new rules, but you may not meet with your care team for several weeks.
The shadow of pregnancy loss may loom over those first months, or you may be worried about pregnancy complications.
Here's what you need to know about symptoms you might experience, as well as fetal development and prenatal visits in the first trimester.
The length of a pregnancy and the three trimesters can be confusing. Does pregnancy last nine months or 40 weeks (10 months)? Is a trimester three months (12 weeks) or 14 weeks?
Let's start with some pregnancy math. For simplicity, doctors calculate the first day of pregnancy as the first day of your last menstruation. Your first trimester technically starts on this date — the first day of your last period. But you don't actually conceive until about two weeks later and won't know you're pregnant for another two weeks after that.
By the time you get a positive pregnancy test or miss your period, you may already be three, four, or more weeks pregnant. And when you're in your fifth week of pregnancy, you're actually four weeks and so many days pregnant. This pregnancy math accounts for the confusing and slightly different definitions of the first trimester.
To keep it simple, UPMC uses the 13-week definition of a trimester from the American College of Obstetricians and Gynecologists (ACOG). According to ACOG, the first trimester ends after the 14th week — when you've passed 13 weeks and six days. You're officially in the second trimester when you're 14 weeks and 0 days pregnant.
Symptoms and timing vary from person to person and pregnancy to pregnancy. Each person may react to pregnancy differently, and each pregnancy can have its own unique symptoms and feelings.
Fatigue is a very common first-trimester symptom. One study by Nature Partner Journals found fatigue most commonly peaks around week seven or eight. However, your experience with fatigue could differ from the average pregnancy – you might feel most tired at week 11 or 12, for example.
If you experience morning sickness, research shows it is usually the worst around week nine. After that, it gets better and often goes away in the second trimester. (For some people with hyperemesis gravidarum, morning sickness doesn't go away until delivery).
The first trimester can be hard with exhaustion and nausea. These may interfere with your daily life. It's also difficult to deal with these symptoms if you're keeping your pregnancy a secret.
The second trimester often is the easiest when it comes to symptoms. During the third trimester, pregnant people often have discomfort, more appointments, and potential complications. The hardest trimester for you will depend on your pregnancy and personal experiences.
The hardest week in the first trimester of pregnancy is often around weeks eight or nine. During these weeks, you may feel very tired and queasy.
Ovulation — when the ovary releases an egg — happens around cycle day 14. You are already two weeks into "pregnancy" at this point. It's during week three of pregnancy that your future baby is created.
Even early pregnancy tests usually don't come back positive until around week four. Early testing may show a very faint positive line around nine days after ovulation, somewhere around day 23 of your cycle.
During this embryonic stage, the sperm fertilizes the egg into a zygote, which grows and divides, travels down the fallopian tube, and implants in the uterus.
Implantation can cause slight vaginal bleeding, or spotting, called implantation bleeding. Because of the timing, this spotting sometimes is confused for a period.
If the embryo fails to implant, you may see a faint line on a pregnancy test. A blood test will show HCG in your system, but the line won't darken, and HCG levels won't increase quickly enough. This is called a very early pregnancy loss or a "chemical pregnancy" that doesn't continue past week five and results in a heavier-than-normal period.
Your embryo is about the size of a poppy seed at week four. It grew from one cell on day 14 to a few hundred cells. If implantation is successful, these cells grow and become the placenta, fluid-filled amniotic sac, umbilical cord, and embryo.
The placenta delivers nutrient-filled blood to your embryo through the umbilical cord. The amniotic sac cushions and protects your embryo and regulates its temperature.
Your embryo's cells multiply very fast, turning into hundreds of different types of cells. The brain, heart, and other major organs begin to take shape. The arms and legs grow in the embryonic stage, although they look like flippers at first. The embryo is not technically a fetus until week 10.
By the time you reach week 10 of pregnancy, your embryo becomes a fetus, measuring about one inch long. The fingers and toes form. All of the organs are present, though very underdeveloped.
From weeks 10 to 13, the fetus grows from one to three inches. By the end of the first trimester, the fetal organs start to function. The fetal heart beats, and the kidneys process waste.
Though you won't feel any movement yet, critical development happens in the first trimester. Compared to later trimesters, the embryo or fetus is more vulnerable. Be sure to take a prenatal vitamin with folic acid daily to support the normal development of the brain and spine.
Ideally, you should take prenatal vitamins and avoid any alcohol and any illicit substances when you're trying to conceive. If you didn't plan your pregnancy, take these steps as soon as you find out you are pregnant. See more tips and tricks for a healthy pregnancy to protect the fetus from toxins and infection.
When you find out you are pregnant, you'll need to pick a women’s health provider for your prenatal care.
You'll start seeing your doctor or midwife about once a month in your first trimester. As your pregnancy progresses, your visits will happen more often. Your prenatal provider also will support you through your birth and postpartum period.
Midwives can provide care for normal, low-risk pregnancies.
You should see an obstetrician if you:
Pregnancy can be an anxious time, so it's important to find someone you trust and feel comfortable with. During your first prenatal visit, make sure your provider takes the time to answer your questions and listen to your concerns. If you don't feel comfortable with your prenatal care provider, you can switch to another one.
Call to set up your first appointment shortly after finding out you're pregnant. You want to make sure your preferred prenatal care provider can accommodate you.
Your first prenatal visit usually happens when you're around eight weeks pregnant. In most cases, that's about four weeks after your missed period and a positive pregnancy test. If you have concerns during this month, including symptoms like vaginal bleeding, severe nausea, or cramping, call your provider — they may have you come in.
The first prenatal visit is usually around 45 minutes, often longer than many later appointments. It will include additional testing, such as a Pap smear, bloodwork, and a urine test.
Your doctor or midwife will ask many questions about your previous pregnancies. your current health, and any health problems that run in your family. A key question is the date of your last period — it is what they use to calculate your due date.
During this visit, your provider will tell you about any changes you should make in your diet and exercise and what foods and activities to avoid. Your first prenatal visit is also a chance for you to ask questions about your symptoms, prenatal testing, and more.
You'll get the following tests at your first prenatal visit:
Usually, you won't have an ultrasound on your first prenatal visit. However, you might have one if you're not sure when you got pregnant due to irregular periods.
You might also get an earlier ultrasound if you have bleeding or cramping. This could be a sign of miscarriage or an ectopic pregnancy — a dangerous condition where the embryo implants outside the uterus.
Early ultrasounds require using a transvaginal probe, which is inserted into the vagina. This early ultrasound can confirm the presence of one or more embryos in the uterus, confirm the embryo is implanted and developing correctly, look for a heartbeat after the fifth week, and help estimate your due date.
Your second prenatal visit will usually occur just before the end of the first trimester, around weeks 11 to 13. This visit and subsequent ones will often start the same. You'll get a blood pressure and weight check. You'll also provide a urine sample so your provider can check for signs of infection and other problems, like high blood sugar.
You will most likely have your first ultrasound around the time of this visit. The ultrasound measures the baby's size, confirms the due date, and looks for any signs of problems with the pregnancy.
At your second prenatal visit, your provider will offer screening tests that look for signs of problems in the fetus. You can decline these tests if you would prefer not to know this information.
The first-trimester screen combines the results of a blood test and an ultrasound to assess your baby's risk of Down Syndrome, trisomy 18, and other health problems.
The noninvasive prenatal test (NIPT) is a blood test. Your placenta releases small amounts of your baby's cells into your bloodstream. Using your blood sample, this test analyzes your baby's DNA to assess their risk of genetic problems and tell their sex.
You may get one or both of these fetal screening tests. Your provider can explain the accuracy of each test and which specific problems each test looks for. If a screening test reveals your fetus is at an increased risk of a birth defect, a diagnostic test is needed to confirm it.
In the first trimester, most symptoms are a result of changing hormones. You may feel these symptoms well before you have a visible baby bump. You may also feel little to no symptoms in the first trimester of your pregnancy.
First-trimester symptoms include:
Call your doctor or midwife if you experience any of the following signs. If your symptoms are serious, go to the emergency department.
First-trimester symptoms that require prompt medical attention include:
By UPMC Editorial Staff. Last reviewed on 2024-09-05.
