The First Trimester

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The Stages of Pregnancy

Pregnancy lasts about nine calendar months (about 40 weeks total). It is typically broken down into three “trimesters” or stages of fetal development. We've provided a brief synopsis of fetal development during each trimester in the discussion below. We've also outlined a few of the more common symptoms women that experience within each trimester. It is likely that you will experience symptoms that do not get mentioned in this document. A more complete summary of the stages of pregnancy can be found in books such as, "What to Expect When You're Expecting", online (e.g.,,, or from your doctor.


The First Trimester (Weeks 1–12)

The first trimester of pregnancy lasts from conception until 12 weeks gestation (pregnancy length). Women usually don't realize they are pregnant until at least two weeks into the first trimester, when they miss their period. During the first trimester, your growing baby is developing at an amazing rate.

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Development. A baby starts out as a few cells (called an "embryo" until it reaches its eighth week, and thereafter a "fetus"), and proceeds to develop the basis for all of the mechanisms it will need within the first trimester. Development begins when the fertilized egg attaches itself to the wall of the uterus in a process called "implantation". If all goes well, the fetus will be approximately 6 to 7.5 cm in length, and weigh a little over 1 ounce by the end of the first trimester.

Some important developments that occur during this stage include:

  • The development of the placenta and umbilical cord. These structures are important for bringing nutrients from the mother to the fetus, and for removing waste products from the fetus back to the mother's body.
  • The development of the amniotic sac, a fluid-filled membrane that cushions the developing fetus.
  • The development of all major organs. The heart begins to beat on or around the 25th day after conception.
  • The development of the neural tube, which goes on to form the brain and spinal cord.
  • The development of limb buds, which are the beginnings of arms and legs.
  • The development of fingers, toes, ears, ankles, wrists, and eyelids. By the end of the first trimester the fetus will have finger and toenails and will have “buds” in its mouth area that will go on to become baby teeth.

The Mother's Experience. Women experience a wide range of symptoms as their bodies undergo some amazing changes during the first trimester. It is important to remember that the symptoms you experience are generally normal, and most will go away as your pregnancy progresses. Some symptoms you may experience during this stage include nausea (sometimes accompanied by actual vomiting), frequent urination, fatigue, food aversion, and breast swelling and tenderness. The two most pronounced (intense) symptoms are nausea and fatigue.

Nausea is commonly called "morning sickness" as many women find that it is worse during the morning. However, some women find they are nauseous all the time, and nausea is possible at any time of day or night. Vomiting should not be a cause of alarm unless it occurs more than four times per day or is bloody. Often, the symptoms of morning sickness are worse when your stomach is empty and your blood sugar is low. Avoid this possibility by eating 6 to 8 small meals per day, maintaining a small amount of food in your stomach at all times. Many women find it helpful to keep crackers or dry cereals by their bed so they can eat something before they get up in the morning (or even in the middle of the night if necessary).

Fatigue is normal during pregnancy because of the physical and emotional demands that being pregnant places on your body. During the first trimester, you may find that you can't make it through the day without a nap, or that you feel tired even after sleeping for 8 hours at night. There are a few things you can do to combat fatigue, including making sure you are eating a balanced diet, getting regular exercise, and paying attention to environmental conditions such as poor lighting or ventilation that might contribute to tiredness. Above all, the most important thing you can do for yourself during this time is to rest when necessary. Your body is working very hard to build your new baby during this stage of your pregnancy; the fatigue you feel is your body's way of telling you it needs rest. You will likely find that your energy levels return to normal as your body gets used to the new demands it faces.

Complications and Miscarriage.  Although most pregnancy symptoms present during the first trimester are benign and merely annoying, there are also a few symptoms to watch out for that will require medical attention when present. Contact your doctor if you experience any of the following symptoms:

  • bloody or watery vaginal discharge
  • edema (swelling of the hands, feet, or face)
  • recurring headaches
  • excessive vomiting
  • blurry vision
  • dizziness
  • fevers
  • decreased urination or burning with urination

Spontaneous abortion (miscarriage) is most common during the first trimester of pregnancy. Miscarriages often occur when an abnormal fetus (for example, a fetus with serious genetic abnormalities that are "incompatible with life") has been conceived, but there are many other reasons why miscarriages may occur. Miscarriages are fairly common events and often are not a reflection of a woman's underlying fertility. Having a single miscarriage doesn't mean you will necessarily have a difficult time conceiving the next time you try and get pregnant (although multiple miscarriages in a row should get you thinking about consulting with an infertility specialist).

Because of the emotions and expectations surrounding pregnancy, miscarriages can be very sad and difficult to come to terms with. If you experience a miscarriage, do what you can to talk about your emotions with trusted family members and other supportive people who can help you through what is generally a difficult time.

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