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.

ANH medical specialists have reviewed this information. It is for educational purposes only and is not intended to replace the advice of your doctor or other health care provider. We encourage you to discuss any questions or concerns you may have with your provider.

Baby’s Month-to-Month Development

First Month 4-8 weeks (since last mentrual period)

The baby has begun to develop a heart, liver and digestive system. The baby is being nourished and getting rid of wastes through the placenta and umbilical cord (the vascular structures that connect baby to the wall of the uterus). The entire baby is approximately 1⁄8 inch (1⁄2 cm) in length.


Second Month 8-12 weeks

By the end of the second month, most women begin to notice the physical signs of pregnancy (i.e. nausea, fatigue, breast pain, etc.). The baby’s arms and legs have begun to form. All the major internal organs have developed and the tiny heart begins to pump blood. Facial features become more defined and brain development is well underway. The baby is nearly 2 inches (5 cm) long.


Third Month 12-16 weeks

By the third month, the baby is now growing rapidly, adding a few millimeters of length each day. Features are becoming distinct. The baby weighs about 1 ounce (28 g) and is 3 inches (8 cm) long.


Fourth Month 16-20 weeks

All of the organs are formed and now the baby must simply grow in size. By the fourth month, babies become more active and may begin to push their arms and legs against the sac in which they float. The baby is now more than 6 inches (15 cm) long and weighs more than 1⁄4 pound (114 g).


Fifth Month 20-24 weeks

Movements are stronger and more easily felt. The baby is now about 10 inches (25 cm) long and weighs about 1⁄2 pound (227 g).


Sixth Month 24-28 weeks

The woman’s abdomen continues to get bigger and the baby’s movements become faster. The baby’s skin is red and wrinkled. The baby is about 12 inches (30 cm) long and weighs about 11⁄2 pounds (689 g).


Seventh Month 28-32 weeks

The baby’s eyes may occasionally be open for short periods of time. If born at this time, the baby would be considered premature and require special care. The baby weighs approximately 21⁄2 pounds (1.13 kg) and is about 15 inches (38 cm) long.


Eighth Month 32-36 weeks

The baby is now almost fully grown and movements or “kicks” are strong enough to see from the outside. The skin is no longer as wrinkled, and the baby is usually in the head-down position from which birth will occur. The baby weighs about 4 pounds (1.81 kg) and is about 161⁄2 inches (42 cm) long.


Ninth Month 36-40 weeks

The baby has now reached a size and maturity that allows it to live outside the mother’s body. The head is covered with hair. The baby settles down lower into the abdomen preparing for birth. The baby weighs around 6 to 7 pounds (2.7 to 3.2 kg) and is 20 inches (50 cm) or more long.


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