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Pregnancy
The Three Trimesters

Pregnancy has three trimesters, each of which is marked by specific fetal developments. A pregnancy is considered full-term at 40 weeks; infants delivered before the end of week 37 are considered premature. Premature infants may have problems with their growth and development, as well as difficulties in breathing and digesting.

First Trimester (0 to 13 Weeks)

The first trimester is the most crucial to your baby's development. During this period, your baby's body structure and organ systems develop. Most miscarriages and birth defects occur during this period.

Your body also undergoes major changes during the first trimester. These changes often cause a variety of symptoms, including nausea, fatigue, breast tenderness and frequent urination. Although these are common pregnancy symptoms, every woman has a different experience. For example, while some may experience an increased energy level during this period, others may feel very tired and emotional.

Second Trimester (14 to 26 Weeks)

The second trimester of pregnancy is often called the "golden period" because many of the unpleasant effects of early pregnancy disappear. During the second trimester, you're likely to experience decreased nausea, better sleep patterns and an increased energy level. However, you may experience a whole new set of symptoms, such as back pain, abdominal pain, leg cramps, constipation and heartburn.

Somewhere between 16 weeks and 20 weeks, you may feel your baby's first fluttering movements.

Third Trimester (27 to 40 Weeks)

You have now reached your final stretch of pregnancy and are probably very excited and anxious for the birth of your baby. Some of the physical symptoms you may experience during this period include shortness of breath, hemorrhoids, urinary incontinence, varicose veins and sleeping problems. Many of these symptoms arise from the increase in the size of your uterus, which expands from approximately 2 ounces before pregnancy to 2.5 pounds at the time of birth.

Reviewed by health care specialists at UCSF Medical Center.

Related Information

UCSF Clinics & Centers

Obstetrics Services at Parnassus
400 Parnassus Ave., Floor B1, Suite A-096
San Francisco, CA 94143
Phone: (415) 353-2566
Fax: (415) 353-2496
Appointment information

Fetal Treatment Center
400 Parnassus Ave., A-123
San Francisco, CA 94143
Phone: (415) 476-0445
Fax: (415) 502-0660
Appointment information

Prenatal Diagnostic Center
350 Parnassus Ave., Suite 810
San Francisco, CA 94117
Phone: (415) 476-4080
Fax: (415) 353-4077
Appointment information

Pregnancy and Postpartum Mood Assessment Clinic
400 Parnassus Ave., Floor B1
San Francisco, CA 94143
Phone: (415) 353-2566
Appointment information

Women's Options Center
2356 Sutter St.
San Francisco, CA 94143-1648
Phone: (415) 353-7003
Fax: (415) 353-9605
Appointment information

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