A woman's body goes through a lot of changes during pregnancy. The first trimester is a time of rapid development for the baby and a lot of internal changes for the mother. Some women experience severe symptoms that last for months, while others have none at all. The first trimester is vital because all the baby's major body systems are beginning to form.

Invisible Changes

A woman's body does not change much on the outside during the first trimester, but plenty is changing inside. As the embryo implants into the wall of the uterus, growth and development happen rapidly. A lot of major changes happen for the mother in the first trimester. They're just not visible.

First Trimester Test katleho Seisa / Getty Images


Early Changes

The amniotic sac, placenta, and umbilical cord are some of the first tissues to form. The placenta is a flat organ that attaches to the uterine wall, where nourishment is passed from mother to baby through the umbilical cord. Inside the umbilical cord, two arteries carry nourishment to the baby and a vein carries waste away. The amniotic sac surrounds the fetus throughout the entire pregnancy. It is filled with amniotic fluid for protection and to help regulate temperature.

Embryo Umbilical Cord luismmolina / Getty Images


Hormone Changes

Hormone surges cause a lot of the symptoms of early pregnancy. Mood swings are common and may be similar to those of premenstrual syndrome. These hormones can also cause nausea and vomiting; this is commonly referred to as morning sickness, although it can occur any time of day. Extreme tiredness is often one of the first signs of pregnancy.

Nausea Morning Sickness PeopleImages / Getty Images


Breast Changes

Women may experience changes to their breasts early in pregnancy. Breasts can swell and ache as progesterone and estrogen production increases. This swelling can also cause veins to become more prominent on the surface of the breasts. Nipple changes can occur, too. Areolas may darken and enlarge, and Montgomery's tubercles may appear. These small, white bumps are enlarged sweat glands.

Breast Changes Early Pregnancy milanvirijevic / Getty Images


Fetal Development — Weeks 1 to 4

During the first four weeks, all the major organs begin to form, including the heart, circulatory system, and digestive system. A neural tube later develops into the brain and spinal cord. Ears and eyes begin their development, and buds grow that will become arms and legs. At this stage, the embryo resembles a tadpole and has a beating heart.

First Four Weeks burakkarademir / Getty Images


Weeks 4 to 8

As the first trimester progresses, the baby starts to take a more human shape, though it has a disproportionately large head. Webbed fingers and toes are now visible on the growing arms and legs. The nervous and urinary systems develop. Tooth buds form and the mouth, nose, and ears take shape. Bones start to develop, and the embryo constantly moves, though the mother cannot feel it.

Human Shape Movement delectus / Getty Images


Weeks 9 to 12

At this point, the baby becomes a fetus rather than an embryo. It is about an inch long, and all the major organs and body systems have formed. Genitalia develops, distinct arms and legs are visible, and the larynx starts to take shape. The fetus now has eyelids, fingernails, and toenails. The mother may be able to feel twitches of movement often referred to as "quickening."

Fully Formed Fetus photosbyhope / Getty Images


The Importance of the First Trimester

The first 12 weeks are crucial for development. Because all the organ systems are just forming, the baby is susceptible to damage from exposure to viruses, drugs, tobacco, radiation, and toxic chemicals. This is one of the main reasons that appropriate prenatal care is essential for a healthy baby.

Essential Prenatal Care FatCamera / Getty Images


The First Prenatal Visit

The first prenatal visit can be quite extensive. In addition to a detailed medical history, the doctor will probably ask about current medications and any preexisting conditions. Issues such as diabetes or high blood pressure might be affected by pregnancy. A family medical history will be taken to discover potential genetic disorders, and a gynecological history helps the doctor learn more about previous pregnancies, deliveries, or miscarriages. The expectant mother will also receive information about diet, exercise, and drug use during pregnancy.

First Prenatal Visit FatCamera / Getty Images


Exam and Labwork

The doctor will also perform a pelvic exam to determine the age of the fetus and the position of the uterus. A pap test can check for any abnormalities. Urine tests are done regularly throughout the pregnancy to check for glucose and protein, which can indicate complications. Another important test is the Rh factor. If the mother's blood is Rh-negative and the fetus' is Rh-positive, the mother may make antibodies which can lead to anemia in the fetus. This issue requires close monitoring throughout the pregnancy.

Pelvic Exam Labs FatCamera / Getty Images


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