The First Trimester

The first prenatal visit

The first prenatal visit is the most thorough. A healthcare provider asks the parent-to-be about their health history and does a physical exam. Certain tests and procedures are also done to check the initial health of the parent and their unborn baby. The first prenatal visit may include:

  • Personal health history. This may cover:

    • Past and current health conditions such as diabetes, high blood pressure (hypertension), anemia, and allergies

    • Current medicines (prescription and over the counter)

    • Past surgeries

  • Maternal and paternal family health history. The healthcare provider will ask about illnesses, such as diabetes or intellectual or developmental disabilities. They will also ask about genetic disorders such as like sickle-cell disease or Tay-Sachs disease.

  • Personal gynecological and obstetrical history. The healthcare provider will want to know about past pregnancies (stillbirths, miscarriage, deliveries, terminations) and menstrual history (length and duration of menstrual periods).

  • Education. The healthcare provider will talk with the pregnant person about the importance of eating a healthy diet, exercising regularly, and not using alcohol, illegal drugs, and tobacco. They will also cover any concerns about domestic violence.

  • Pelvic exam. This exam may be done for one or all these 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. These may include:

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

    • Blood tests. These are done to determine blood type. All pregnant people are tested for the Rh factor during the early weeks of pregnancy. A parent and fetus may have incompatible blood types. The most common is Rh incompatibility. Rh incompatibility happens when the pregnant person's blood is Rh-negative and the partner's blood is Rh-positive and the fetus' blood is Rh-positive. The pregnant person may make antibodies against the Rh-positive fetus, which may lead to anemia in the fetus. Incompatibility problems are closely watched. Treatment is available to prevent the formation of Rh antibodies during pregnancy.

  • Blood screening tests. These are done to find diseases such as rubella. This infectious disease is also called German measles.

  • Genetic tests. These are done to find inherited diseases such as sickle-cell anemia and Tay-Sachs disease.

  • Screening tests. These are done to find infectious diseases such as sexually transmitted infections.

The first prenatal visit is also a good time for the parent-to-be to ask any questions or talk about any pregnancy concerns.

What to expect during the first trimester

A healthy first trimester is crucial to the normal development of the fetus. The parent-to-be may not be showing much on the outside. But inside their 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:

  • Amniotic sac. A sac filled with amniotic fluid 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 pregnant person's blood. The fetal blood vessels are separated from the person'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 2 arteries and a vein. These 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 prone to damage from substances like alcohol, illegal drugs, certain medicines, and illnesses, such as rubella.

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

Side view of female body showing reproductive system and inset of 3 month fetus.

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. By the end of the first trimester, it becomes a fetus that is fully formed. The fetus weighs about 1/2 to 1 ounce and measures, 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. But 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. But 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. 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 parent.

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.

  • The fetus starts to move more.

  • 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 time, all the major organs and body systems are forming. They can be damaged if the fetus is exposed to drugs, rubella, 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 pregnant person's body

During pregnancy, many changes are also happening in the body of the parent-to-be. People experience these changes differently. Some symptoms of pregnancy continue for several weeks or months. Others are only experienced for a short time. Some people have many symptoms. Others have only a few or none at all. Here 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 person's areolas (the pigmented areas around each breast's nipple) will enlarge and darken. They may become covered with small, white bumps. These are 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 pregnant person's bladder. They may need to urinate more often.

  • Partly due to surges in hormones, a pregnant person may have mood swings much like premenstrual syndrome (PMS). People with PMS may have 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. Morning sickness does not necessarily happen just in the morning. And it rarely interferes with correct nutrition for the pregnant person 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 because of 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 increases for the growing fetus.

  • A pregnant person may be extremely tired due to the physical and emotional demands of pregnancy.

  • Blood volume increases by about 40% to 50% from the start to the end of the pregnancy. It causes an increased output from the heart. An increased heart output may cause a higher pulse rate during pregnancy. The increase in blood volume is needed for extra blood flow to the uterus.

Online Medical Reviewer: Donna Freeborn PhD CNM FNP
Online Medical Reviewer: Heather M Trevino BSN RNC
Online Medical Reviewer: Irina Burd MD PhD
Date Last Reviewed: 1/1/2023
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