The first trimester is the first 13 weeks of pregnancy. It is a period of rapid change in both the mother and the developing baby. For the mother, pregnancy hormones begin producing noticeable physical and emotional effects, while the uterus starts changing to support pregnancy. For the baby, the fertilized egg becomes an embryo, implants in the uterus, and develops the basic structures that will later become the organs and body systems.
Although the embryo is very small during these weeks, development is remarkably active. Many of the changes that establish the body’s basic organization occur during the first trimester.
What happens at the beginning of pregnancy
Pregnancy begins when a sperm fertilizes an egg, forming a single cell called a zygote. The cell divides repeatedly as it moves toward the uterus. After several days, it develops into a structure called a blastocyst.
The blastocyst attaches to and begins embedding itself in the lining of the uterus, a process called implantation. Cells that will become the embryo begin separating from cells that will contribute to structures supporting the pregnancy, including the placenta.
Once implantation occurs, pregnancy hormones rise. One of the earliest important hormones is human chorionic gonadotropin, or hCG. Pregnancy tests detect hCG in urine or blood.
How the baby develops during the first trimester
During the earliest weeks, the developing baby is called an embryo. Around the ninth week of pregnancy, the developing baby is generally referred to as a fetus.
The first trimester is especially important because the embryo’s basic body plan is established. Cells become specialized and organize into tissues and organs. The nervous system begins forming, the heart develops, and the foundations of the limbs and other major structures appear.
The neural tube, which develops into the brain and spinal cord, forms early in embryonic development. The heart and blood vessels also begin developing early, allowing the growing embryo to establish its own circulation.
The arms and legs begin as small buds. Over time, these structures become more defined, with developing hands, feet, fingers, and toes.
Facial development also begins during this period. Structures that will form the eyes, ears, nose, mouth, and other facial features gradually take shape as different groups of cells grow, move, and join together.
By the end of the first trimester, the fetus has recognizable human features, although it is still very small. Many organs and body systems have begun developing, but they are not yet fully mature.
What happens to the mother’s body
The mother’s body begins adapting to pregnancy soon after implantation. These changes are largely driven by hormones and are necessary to support the developing pregnancy.
Progesterone helps maintain the uterine lining and supports the conditions needed for pregnancy. Estrogen levels also increase and contribute to changes in the uterus, breasts, and blood circulation.
The uterus begins enlarging as the pregnancy progresses. The cervix and surrounding tissues also undergo changes that help support the pregnancy.
At the same time, the breasts begin preparing for milk production. Increased blood flow and hormonal changes can make the breasts swollen, tender, or unusually sensitive.
These changes can begin before the pregnancy is visibly noticeable.
Why morning sickness happens
Nausea and vomiting are common during the first trimester, although not everyone experiences them. Despite the name “morning sickness,” nausea can occur at any time of day.
The exact cause is not completely understood, but changing pregnancy hormones are an important part of the process. Hormonal changes can affect the digestive system and the brain regions involved in nausea and vomiting.
For many pregnant people, nausea becomes noticeable during the early weeks and improves later in pregnancy. Some experience only mild nausea, while others have more persistent symptoms.
Severe vomiting that makes it difficult to keep fluids down can cause dehydration and requires medical attention.
Why fatigue is so common
Strong fatigue is another common feature of early pregnancy. Rising progesterone levels can contribute to sleepiness, while the body is also undergoing major physiological changes.
The cardiovascular system begins adapting to pregnancy, and the body starts increasing the resources needed to support the uterus, placenta, and developing baby. These adjustments can leave a person feeling unusually tired even when daily activities have not changed.
Fatigue often becomes less intense later in pregnancy, although energy levels vary from person to person.
Other common first-trimester changes
Hormonal and physical changes can produce many symptoms besides nausea and fatigue. A pregnant person may notice more frequent urination, changes in appetite or food preferences, increased sensitivity to smells, bloating, constipation, headaches, or mood changes.
Frequent urination can occur partly because pregnancy changes blood flow through the kidneys and increases the amount of fluid processed by the body. As pregnancy progresses, the growing uterus can also contribute to pressure on the bladder.
Digestive changes are influenced in part by hormones such as progesterone, which can slow movement through the digestive tract. This can contribute to constipation and bloating.
Not everyone experiences the same symptoms, and the presence or absence of a particular symptom does not by itself indicate how a pregnancy is progressing.
How the placenta develops
The placenta is a temporary organ that develops during pregnancy and connects the mother and developing baby through the umbilical cord.
During the first trimester, the placenta develops rapidly. It becomes the major interface for exchanging oxygen, nutrients, and waste products between the mother’s blood and the developing baby’s circulation.
The placenta also produces hormones that help maintain pregnancy and support changes in the mother’s body.
Maternal and fetal blood normally remain in separate circulatory systems. Substances can move between them across specialized placental tissues without the two blood supplies simply mixing together.
What the baby can do by the end of the first trimester
By the end of the first trimester, development has progressed far beyond the earliest stage of a cluster of dividing cells. The fetus has developing arms and legs, fingers and toes, facial features, and major internal organs.
The heart is functioning, and movement begins during the first trimester. However, early fetal movements are generally too subtle for the pregnant person to feel.
The brain and nervous system continue developing rapidly. Muscles and nerves are becoming increasingly organized, allowing the fetus to make small movements.
Although the major foundations of many organs have been established, development is far from complete. The second and third trimesters involve substantial growth, maturation, and refinement of organs and body systems.
Why the first trimester is especially sensitive
The first trimester is a particularly important period because many organs and body structures are being established. During this stage, disruption of normal development can have a greater effect on the formation of certain structures than it would later in pregnancy.
This is one reason healthcare professionals pay close attention to medications, infections, alcohol, tobacco, certain chemicals, and other potential exposures during early pregnancy. The effect of an exposure depends on factors such as what the substance is, how much exposure occurred, when it occurred, and individual circumstances.
Because pregnancy can involve significant changes in medication needs and safety, medications and supplements should be discussed with a healthcare professional rather than started or stopped without guidance.
How the first trimester ends
By the end of the first trimester, the pregnancy has undergone a major transition. The embryo has developed into a fetus with the basic structures of the body established, while those structures continue growing and maturing.
Meanwhile, the mother’s body has adapted to sustain the pregnancy through extensive hormonal, circulatory, reproductive, and metabolic changes.
The first trimester therefore combines two closely connected processes: rapid construction of the developing baby’s body and equally important adaptation of the mother’s body to support it.