The First Trimester

The first prenatal visit

The first prenatal visit is the most thorough. A complete medical history is taken, a physical exam is done, and certain tests and procedures are performed to assess the initial health of the mother and her unborn baby. The first prenatal visit may include the following:

  • Personal medical history. This may include taking record of any of the following:

    • Previous and current medical conditions, like diabetes, high blood pressure (hypertension), anemia, and/or allergies

    • Current medicines. Prescription and over-the-counter.

    • Previous surgeries

  • Maternal and paternal family medical history, including illnesses, such as diabetes or intellectual or developmental disabilities, and genetic disorders, like sickle cell disease or Tay-Sachs disease

  • Personal gynecological and obstetrical history, including past pregnancies—stillbirths, miscarriage, deliveries, terminations—and menstrual history (length and duration of menstrual periods)

  • Education, including a discussion regarding the importance of proper nutrition, regular exercise, the avoidance of alcohol, drugs, and tobacco during pregnancy, and a discussion of any concerns about domestic violence

  • Pelvic exam. This exam may be done for 1 or all of the following reasons:

    • To note the size and position of the uterus

    • To determine the age of the fetus

    • To check the pelvic bone size and structure

    • To perform a Pap test (also called Pap smear) to find the presence of abnormal cells

  • Lab tests, including the following:

    • Urine tests. These are done to screen for bacteria, sugar, and protein.

    • Blood tests. These are done to determine blood type.

      All pregnant women are tested for the Rh factor during the early weeks of pregnancy. A mother and fetus may have incompatible blood types. The most common is Rh incompatibility. Rh incompatibility happens when the mother's blood is Rh-negative and the father's blood is Rh-positive and the fetus' blood is Rh-positive. The mother may make antibodies against the Rh-positive fetus, which may lead to anemia in the fetus. Incompatibility problems are watched and appropriate medical treatment is available to prevent the formation of Rh antibodies during pregnancy.

  • Blood screening tests. These are done to find diseases, like rubella, an infectious disease that is also called German measles.

  • Genetic tests. These are done to find inherited diseases, like sickle-cell anemia, Tay-Sachs disease.

  • Screening tests. These are done to find infectious diseases, like sexually transmitted diseases.

The first prenatal visit is also an opportunity to ask any questions or discuss any concerns that you may have about your pregnancy.

What to expect during the first trimester

A healthy first trimester is crucial to the normal development of the fetus. The mother-to-be may not be showing much on the outside, but inside her body all the major body organs and systems of the fetus are forming.

As the embryo implants itself into the uterine wall, several developments take place, including:

  • Amniotic sac. A sac filled with amniotic fluid, called the amniotic sac, surrounds the fetus throughout the pregnancy. The amniotic fluid is liquid made by the fetus and the amnion (the membrane that covers the fetal side of the placenta) that protects the fetus from injury. It also helps to regulate the temperature of the fetus.

  • Placenta. The placenta is an organ shaped like a flat cake that only grows during pregnancy. It attaches to the uterine wall with tiny projections called villi. Fetal blood vessels grow from the umbilical cord into these villi, exchanging nourishment and waste products with the mother's blood. The fetal blood vessels are separated from the mother's blood supply by a thin membrane.

  • Umbilical cord. The umbilical cord is a rope-like cord connecting the fetus to the placenta. The umbilical cord contains two arteries and a vein, which carry oxygen and nutrients to the fetus and waste products away from the fetus.

It is during this first trimester that the fetus is most susceptible to damage from substances, like alcohol, drugs, certain medicines, and illnesses, like rubella (German measles).

During the first trimester, both the mother's body and the fetus are changing rapidly.

Illustration demonstrating fetal growth from 8 to 40 weeks
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Fetal development during the first trimester

The most dramatic changes and development happen during the first trimester. During the first 8 weeks, a fetus is called an embryo. The embryo develops rapidly and by the end of the first trimester it becomes a fetus that is fully formed, weighing approximately 1/2 to 1 ounce and measuring, on average, 3 to 4 inches in length.

First trimester growth and development benchmarks

Just as each child grows and matures at different rates and at different times, so does that same child as it begins its life in the womb. The chart below provides benchmarks for most normal pregnancies. However, each fetus develops differently.

By the end of 4 weeks

  • All major systems and organs begin to form

  • The embryo looks like a tadpole

  • The neural tube (which becomes the brain and spinal cord), the digestive system, and the heart and circulatory system begin to form

  • The beginnings of the eyes and ears are developing

  • Tiny limb buds appear (which will develop into arms and legs)

  • The heart is beating

By the end of 8 weeks

  • All major body systems continue to develop and function, including the circulatory, nervous, digestive, and urinary systems

  • The embryo is taking on a human shape, although the head is larger in proportion to the rest of the body

  • The mouth is developing tooth buds (which will become baby teeth)

  • The eyes, nose, mouth, and ears are becoming more distinct

  • The arms and legs can be easily seen

  • The fingers and toes are still webbed, but can be clearly distinguished

  • The main organs continue to develop and you can hear the baby's heartbeat using an instrument called a Doppler

  • The bones begin to develop and the nose and jaws are rapidly developing

  • The embryo is in constant motion but cannot be felt by the mother

From embryo to fetus

After 8 weeks, the embryo is now referred to as a fetus (which means offspring).

Although the fetus is only 1 to 1 1/2 inches long at this point, all major organs and systems have been formed.

During weeks 9 to 12

  • The external genital organs are developed

  • Fingernails and toenails appear

  • Eyelids are formed

  • Fetal movement increases

  • The arms and legs are fully formed

  • The voice box (larynx) begins to form in the trachea

The fetus is most vulnerable during the first 12 weeks. During this period of time, all of the major organs and body systems are forming and can be damaged if the fetus is exposed to drugs, German measles, radiation, tobacco, and chemical and toxic substances.

Even though the organs and body systems are fully formed by the end of 12 weeks, the fetus cannot survive independently.

Changes in the mother's body

During pregnancy, many changes are also happening in the body of the mother-to-be. Women experience these changes differently. Some symptoms of pregnancy continue for several weeks or months. Others are only experienced for a short period of time. Some women experience many symptoms. Other women experience only a few or none at all. The following is a list of changes and symptoms that may happen during the first trimester:

  • The mammary glands enlarge causing the breasts to swell and become tender in preparation for breastfeeding. This is due to an increased amount of the hormones estrogen and progesterone. A supportive bra should be worn.

  • A woman's areolas (the pigmented areas around each breast's nipple) will enlarge and darken. They may become covered with small, white bumps called Montgomery tubercles (enlarged sweat glands).

  • Veins become more noticeable on the surface of the breasts.

  • The uterus is growing and begins to press on the woman's bladder. This causes her to need to urinate more often.

  • Partly due to surges in hormones, a pregnant woman may experience mood swings similar to premenstrual syndrome. This is a condition experienced by some women that is characterized by mood swings, irritability, and other physical symptoms that happen shortly before each menstrual period.

  • Increased levels of hormones to sustain the pregnancy may cause "morning sickness." This is feelings of nausea and sometimes vomiting. However, morning sickness does not necessarily happen just in the morning and rarely interferes with proper nutrition for the mother and her fetus.

  • Constipation may happen as the growing uterus presses on the rectum and intestines.

  • The muscular contractions in the intestines, which help to move food through the digestive tract, are slowed due to high levels of progesterone. This may, in turn, cause heartburn, indigestion, constipation, and gas.

  • Clothes may feel tighter around the breasts and waist, as the size of the stomach begins to increase to accommodate the growing fetus.

  • A woman may experience extreme tiredness due to the physical and emotional demands of pregnancy.

  • Cardiac volume increases by about 40% to 50% from the beginning to the end of the pregnancy. This causes an increased cardiac output. An increased cardiac output may cause an increased pulse rate during pregnancy. The increase in blood volume is needed for extra blood flow to the uterus.

Online Medical Reviewer: Daniel N Sacks MD
Online Medical Reviewer: Donna Freeborn PhD
Online Medical Reviewer: Heather Trevino
Date Last Reviewed: 1/1/2023
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