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  T. Tri Week by Week
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Get to Know the Trimesters – Understanding Your Baby's Developmental Stages

In general, the average pregnancy lasts 40 weeks and is divided into three periods of time, called trimesters. The first trimester lasts until week 13, the second trimester until week 28, and the third trimester lasts until the baby is born.

During the first trimester, there’s a lot going on in your new baby’s life. The baby will grow from a fertilized egg to a blastocyst, and then to an embryo before becoming a fetus. From a small cluster of undifferentiated cells, a baby will form, complete with head, brain, eyes, arms, legs, fingers and toes, organs and a beating heart. During the first trimester, the umbilical cord and placenta also develop.

The first period is sometimes challenging for the mother-to-be. As a result of all the changes taking place, she may experience morning sickness, breast fullness and tenderness, constipation, dizziness, spotting, uterine cramping, mood swings, heartburn, indigestion, frequent urination, night sweats and round ligament pain. While many of these symptoms disappear after the first trimester, they’ll certainly make life uncomfortable for a few months.

Much of the baby’s development is completed by the end of the first trimester, so the second trimester is more of a time for continued refinement and growth. During this stage, the baby will be covered with protective vernix and lanugo, and mom will finally begin to feel her baby moving around inside the uterus. In the second trimester, the baby will grow to a weight of about two pounds and begin cycles of waking and sleeping, much like a newborn.

The second trimester is usually easier for the mother-to-be. Many of the earlier symptoms of pregnancy she experienced will decrease or end all together as her hormone levels stop climbing and begin to level off. For many women the “magic middle” months are very pleasant. The symptoms of pregnancy are more manageable, yet the mom-to-be hasn’t yet gained so much weight that it’s physically difficult for her to do things.

During the third trimester, the baby's last organ to develop will complete its preparation for birth. That organ is the lungs, and one of the last things they’ll do near term is to begin to produce surfactant that will prepare the baby for breathing on her own. From week 30, the baby will gain about one-half pound each week to reach her final birth weight of an average of eight pounds. Around week 36, the baby will usually move into the birth canal and settle into position for birth.

As birth approaches, the mother-to-be will be experiencing Braxton Hicks contractions, which help prepare the uterus for delivery. She will continue to gain weight, and as her uterus presses against her rib cage, she may have difficulty breathing. As she nears term, however, the uterus will drop forward, and she’ll find it easier to breathe. Unfortunately, though, her weight gain may cause her to have back aches and she may experience swelling in her feet and legs. All of these will be relieved with the delivery of the baby, which will happen between weeks 40-42.

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