The first trimester is the first third of pregnancy, and it lasts roughly 3 months. Most doctors define it as weeks 1–12 of pregnancy, but some say that it extends into week 14.

Doctors count from the first day of a person’s last menstrual period to determine the stage of a pregnancy.

By the time a person misses their period, they are already about 4 weeks pregnant, even though conception occurred only a week or two earlier.

Therefore, a person typically only knows that they are pregnant for about 2 months of the first trimester.

This article defines and describes the first trimester of pregnancy. It also outlines the symptoms that people may experience and explains what to avoid in the first trimester.

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It is a personal decision when to tell others, including friends and family, about a pregnancy.

Some people may want to reveal that they are pregnant as soon as they find out, whereas others may prefer to wait a few weeks. There is no right way to reveal a pregnancy, and the timing depends on a person’s comfort level.

Many people delay telling others that they are pregnant until the end of the first trimester. The reason for this is that pregnancy loss rates are much lower in the second trimester and beyond.

However, a person may still wish to see a healthcare provider about their pregnancy within a few weeks of finding out that they are pregnant. The healthcare provider can help ensure that they have a healthy pregnancy.

Although pregnancy symptoms can be quite intense during the first trimester, most people are not visibly pregnant until at least the end of the first trimester. In some cases, the pregnancy may not be visible to others until much later.

Every pregnancy is different, and some people experience no pregnancy symptoms at all.

For those who do notice changes, the most common symptoms include:

  • Fatigue: People may feel more tired than usual, with some individuals experiencing intense exhaustion.
  • Nausea: Nausea is one of the most common pregnancy symptoms. If nausea causes frequent vomiting or makes it impossible to eat or keep down food, it is important to call a healthcare provider.
  • Aches and pains: Some people experience headaches or other muscle aches and pains.
  • Breast changes: The breasts may appear fuller or change appearance in other ways. They may become very tender, and some people notice increased nipple sensitivity.
  • Changes in mood: Pregnancy represents a huge change in a person’s life, and they may feel many different emotions about it. Pregnancy hormones and the physiological discomforts of pregnancy may also affect mood.
  • Dizziness: Some people feel very dizzy, especially when they are hungry or thirsty or stand up too quickly.
  • Nasal congestion: Changes in blood flow in the body may cause increased sinus congestion or pressure. Some people have nosebleeds.
  • Vaginal discharge changes: A person’s usual pattern of discharge may change. They may have more discharge or notice that it has a different texture. It is not normal for the vagina to hurt or itch, so anyone experiencing these symptoms should call a doctor.
  • Spotting: Some people experience light bleeding, especially early on in the first trimester. A person should call a doctor if the bleeding is heavy or dark red.

In most cases, a person does not need extra calories during the first trimester, and weight gain is minimal.

The fertilized egg develops into an embryo, which becomes a fetus 8 weeks into the pregnancy.

During the first 8 weeks of pregnancy, a rudimentary brain and spine form, along with muscles of the eyes, nose, and mouth. The fetus measures 1–1.5 inches (in), or 2.5–3.8 centimeters (cm), at 8 weeks.

By the end of the first trimester, all major organ systems have formed, although the organs are not fully developed.

Muscles and bones are beginning to form, and the external genitals are growing. An ultrasound cannot detect the sex of the fetus at this early stage, but a blood test may.

The fetus is larger at the end of the first trimester — about 3–4 in (7.6–10.2 cm) long. Although the fetus starts moving in the first trimester, it is usually too small for the pregnant person to detect any movement.

However, some pregnant people report feeling movement very early, especially if they have been pregnant before.

The first trimester is the most vulnerable time for fetal development because all of the major organs are forming at this time.

The risk of pregnancy loss drops significantly in the second trimester.

A person may not look visibly pregnant in the first trimester. However, if they choose to continue the pregnancy, they should consider making healthful diet and lifestyle choices during this time to maximize the chances of a healthy pregnancy and newborn.

If birth abnormalities happen, they tend to occur early in development. Healthful choices during the first trimester, such as taking folic acid supplements and avoiding smoking, can help reduce the chance of these abnormalities.

A person can talk to a doctor for specific recommendations.

The Centers for Disease Control and Prevention (CDC) recommend that most pregnant people take several steps to maintain the health of the pregnancy and fetus. These include:

  • scheduling an appointment with a doctor or midwife
  • talking to a healthcare provider about any chronic medical conditions or current medications
  • starting to build a pregnancy support network, plan for parenthood, and discuss financial concerns
  • taking a daily prenatal vitamin that contains 400 micrograms of folic acid
  • eating a balanced, healthful diet and listening to hunger cues, as there is no need to “eat for two”
  • continuing to exercise, as pregnancy-appropriate exercise can reduce the risk of a number of pregnancy complications

People who did not exercise before becoming pregnant but wish to start increasing their physical activity levels should talk to a healthcare provider.

During pregnancy, each person has a different combination of protective factors and risk factors.

Therefore, a pregnant person should talk to a healthcare provider about how they can stay safe and healthy.

Most people should avoid:

  • skin care products containing retinol or isotretinoin, which may include many acne and anti-aging products
  • alcohol and recreational drugs
  • smoking
  • mosquito bites, as mosquitoes can carry dangerous infections
  • any food that may carry dangerous bacteria, including undercooked meat, raw shellfish, undercooked eggs, and unpasteurized milk and cheese
  • fish that may contain mercury or pollutants, such as shark, swordfish, and marlin
  • changing cat litter, as it poses the risk of toxoplasmosis

It is safe to eat tuna, salmon, sardines, and some other oily fish.

The Food and Drug Administration (FDA) recommend that pregnant people eat 8–12 ounces (oz) of low-mercury seafood per week, which equates to two to three 4-oz servings.

Some research suggests that caffeine may increase the risk of pregnancy loss, especially in people who consume it in high amounts. More recent research suggests that even moderate caffeine consumption may be harmful. Thus, research results are mixed and inconclusive.

To reduce the risk, it is advisable to drink less caffeine. For most pregnant people, less than 200 milligrams of caffeine per day may be safe.

However, a person should talk to a doctor about coffee and other food and drinks that contain caffeine.

They should also talk to the doctor about any medications or supplements that they take.

It is essential not to stop taking prescription drugs without talking to a doctor first.

As soon as a person learns that they are pregnant, it is a good idea to call a doctor to schedule a prenatal appointment.

A doctor may schedule an early ultrasound to ensure that the pregnancy is in the uterus and that the fetus is developing normally.

Early blood work may help detect common issues, such as thyroid disease or sexually transmitted infections (STIs) that might affect the fetus.

A person should call a doctor if they:

  • get intense headaches
  • cannot stop vomiting
  • experience dizziness that is intense enough to interfere with their daily functioning
  • have a fever or intense cramps in their stomach
  • begin to bleed heavily

The first trimester of pregnancy may feel like an uncertain time, but a strong relationship with a trusted healthcare provider can help ease anxiety.

These professionals can help ensure the health of the person and their pregnancy and offer practical strategies for dealing with early pregnancy symptoms.