Pregnancy First Trimester

Pregnancy: First Trimester

Information is provided on what you can expect during the first 3 months of your pregnancy. The first section below looks at development during the each of the first 3 months and the second section considers development week-by-week instead of month-by-month.

Pregnancy First Trimester

The First Trimester: An Overview

During the first 3 months of pregnancy, called the first trimester, your body undergoes many changes. As your body adjusts to the growing baby you may have nausea, fatigue, backaches, mood swings, and stress. These things are all normal and most of these discomforts will go away as your pregnancy progresses.

Some of the most common changes or symptoms you might experience in the first trimester include:


Many women experience fatigue and exhaustion in the first trimester. This is one of your body's normal ways of reacting to hormonal changes that are taking place in early pregnancy. To help ease the fatigue try to get at least eight hours of sleep every night and a nap during the day when possible.  For other suggestions visit the page fatigue during pregnancy.

Morning Sickness

Nausea and vomiting, usually called "morning sickness," are common during early pregnancy. However, for many women it is not limited to just the morning and often appears at other times.  The nausea and vomiting usually go away after the first trimester. For additional information please visit the morning sickness link.

When you vomit you lose body fluid.  If you lose too much fluid you can become dehydrated which can be dangerous for you and your baby. If you think you are vomiting excessively call your doctor.

Frequent Urination

Early in pregnancy the growing uterus presses on your bladder and this can cause frequent urination.  Make sure to contact your health care provider if you notice pain, burning, pus or blood in your urine.

Weight Gain

During the first trimester, it is normal for you to have weight gain, about one pound per month for the first 3 months.

Changes in your baby

By the end of the first trimester your baby is about 3 inches long and weighs about half an ounce. The eyes move closer together into their positions, and the ears also are in position. The liver is making bile, and the kidneys are secreting urine into the bladder. Even though you can not feel your baby move yet, your baby will move inside you in response to pushing on your abdomen. Visit the web page Body Changes for additional information.

Medical Care and Prenatal Care

During the early months of pregnancy, regular doctor or midwife visits (prenatal care) are especially important. During the first prenatal visit, you can expect your doctor or midwife to do the following:

  • ask about your health history including diseases, operations, or prior pregnancies

  • obtain your family's health history

  • do a complete physical exam

  • complete a pelvic exam with a Pap test

  • check your blood pressure, urine, and weight and order lab tests

  • determine your expected due date.

Fetal Development: The First Trimester Week-by-Week

Take care of yourself and your baby.

Take a prenatal vitamin or folic acid supplement.
Maintain a healthy weight.
Exercise regularly, with your health care provider's OK
Maintain a healthy diet.
See your health care provider for regular prenatal checkups.
Talk to your health care provider about any medications you are taking.

Do not:
Drink alcohol.
Take medication, herbs, or vitamins without checking with your health care provider.
Use recreational drugs.

For more information about pregnancy do's and don'ts, click on this pregnancy do's and don'ts link.  Click on the following two links for more information about the 2nd trimester and the 3rd trimester of your pregnancy.

The following, adapted from the above Mayo Clinic website, will give you an idea of what is happening during your first 12 weeks of pregnancy.

Week 1: Getting ready

You are not actually pregnant the first week or two of the time allotted to your pregnancy. Conception typically occurs about two weeks after your period begins. To calculate your due date, your health care provider will count ahead 40 weeks from the start of your last period. This means your period is counted as part of your pregnancy even though you were not pregnant at the time.

Week 2: Fertilization

The sperm and egg unite in the fallopian tube to form a one-celled entity called a zygote. If more than one egg is released and fertilized, you may have multiple zygotes. The zygote has 46 chromosomes, 23 from the egg and 23 from the sperm. These chromosomes contain genetic material that will determine your baby's sex and traits such as eye color, hair color, height, facial features and, at least to some extent, intelligence and personality.

Soon after fertilization, the zygote will travel down one of your fallopian tubes toward the uterus. At the same time, it will begin dividing rapidly to form a cluster of cells resembling a tiny raspberry. The inner group of cells will become the embryo. The outer group of cells will become the membranes that nourish and protect it.

Week 3: Implantation

By this time the zygote is made up of about 500 cells and is now known as a blastocyst. When it reaches your uterus, the blastocyst will burrow into the uterine wall for nourishment. The placenta, which will nourish your baby throughout the pregnancy, also begins to form.  By the end of this week, you may test positive in a pregnancy test.

Week 4: The embryonic period begins

The fourth week marks the beginning of the embryonic period, when the baby's brain, spinal cord, heart and other organs begin to form. Your baby is now 1/25 of an inch long.

The embryo is now made of three layers. The top layer, the ectoderm, will give rise to a groove along the midline of your baby's body. This will become the neural tube, where your baby's brain, spinal cord, spinal nerves and backbone will develop.

Your baby's heart and a primitive circulatory system will form in the middle layer of cells, the mesoderm. This layer of cells will also serve as the foundation for your baby's bones, muscles, kidneys and much of the reproductive system.

The inner layer of cells, the endoderm, will become a simple tube lined with mucous membranes. Your baby's lungs, intestines and bladder will develop here.

Week 5: Baby's heart begins to beat

At week five, your baby is 1/17 of an inch long, about the size of the tip of a pen.

This week, your baby's heart and circulatory system are taking shape. Your baby's blood vessels will complete a circuit, and his or her heart will begin to beat. Although you will not be able to hear it yet, the motion of your baby's beating heart may be detected with an ultrasound exam.

With these changes, circulation begins, making the circulatory system the first functioning organ system.

Week 6: The neural tube closes

Growth is rapid this week. Just four weeks after conception, your baby is about 1/8 of an inch long. The neural tube along your baby's back is now closed, and your baby's heart is beating with a regular rhythm.

Basic facial features will begin to appear, including an opening for the mouth and passageways that will make up the inner ear. The digestive and respiratory systems begin to form as well.

Small blocks of tissue that will form your baby's connective tissue, ribs and muscles are developing along your baby's midline. Small buds will soon grow into arms and legs.

Week 7: The umbilical cord appears

Seven weeks into your pregnancy, your baby is 1/3 of an inch long, a little bigger than the top of a pencil eraser. He/she weighs less than an aspirin tablet.

The umbilical cord is the link between your baby and the placenta and it is now clearly visible. The cavities and passages needed to circulate spinal fluid in your baby's brain have formed, but your baby's skull is still transparent.

The arm bud that sprouted last week now resembles a tiny paddle. Your baby's face takes on more definition this week, as a mouth perforation, tiny nostrils and ear indentations become visible.

Week 8: Baby's fingers and toes form

Eight weeks into your pregnancy, your baby is just over 1/2 of an inch long.

Your baby will develop webbed fingers and toes this week. Wrists, elbows and ankles are clearly visible, and your baby's eyelids are beginning to form. The ears, upper lip and tip of the nose also become recognizable.

As your baby's heart becomes more fully developed, it will pump at 150 beats a minute, about twice the usual adult rate.

Week 9: Movement begins

Your baby is now nearly 1 inch long and weighs a bit less than 1/8 of an ounce. The embryonic tail at the bottom of your baby's spinal cord is shrinking, helping him or her look less like a tadpole and more like a developing person.

Your baby's head, which is nearly half the size of his or her entire body, is now tucked down onto the chest. Nipples and hair follicles begin to form. Your baby's pancreas, bile ducts, gallbladder and anus are in place. The internal reproductive organs, such as testes or ovaries, start to develop.

Your baby may begin moving this week, but you will not be able to feel for it quite a while yet.

Week 10: Neurons multiply

By now, your baby's vital organs have a solid foundation. The embryonic tail has disappeared completely, and your baby has fully separated fingers and toes. The bones of your baby's skeleton begin to form.

This week, your baby's brain will produce almost 250,000 new neurons every minute.

Your baby's eyelids are no longer transparent. The outer ears are starting to assume their final form, and tooth buds are forming as well. If your baby is a boy, his testes will start producing the male hormone testosterone.

Week 11: The baby's sex organs may be visible

From now until your 20th week of pregnancy, the halfway mark, your baby will increase his or her weight 30 times and will about triple in length. To make sure your baby gets enough nutrients, the blood vessels in your placenta are growing larger and multiplying.

Your baby is now officially described as a fetus. Your baby's ears are moving up and to the side of the head this week. By the end of the week, your baby's external genitalia will develop into a recognizable penis or clitoris and labia majora.

Week 12: Baby's fingernails and toenails appear

Twelve weeks into your pregnancy, your baby is nearly 3 inches long and weighs about 4/5 of an ounce.

This week marks the arrival of fingernails and toenails. Your baby's chin and nose will become more refined as well.

Additional Information

WebMed: The 1st Trimester of Pregnancy

The First Trimester to-do List

Women's Health: Pregnancy weeks 1-12

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