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What is the Prenatal Period?

By Meshell Powell
Updated Mar 03, 2024
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The term prenatal period refers to the time in pregnancy between conception and birth. It lasts for 40 weeks, which is roughly nine months. It is divided into three trimesters, with each trimester lasting approximately three months. During this time, the embryo develops into a fetus and then into a viable human baby. There are many changes in both mother and child during the prenatal period, including weight gain in both and physical and hormonal changes in each.

During the first trimester of the prenatal period, a woman goes through many physical and emotional changes. This period generally begins with a missed period, which leads to a positive pregnancy test. During the first trimester, the doctor will establish a due date for the baby and may perform an ultrasound to ensure proper fetal development. The soon-to-be mother will likely experience morning sickness and food cravings during the first trimester. Hormonal changes may cause mood changes such as crying easily or extreme euphoria.

Fetal development during the first trimester is remarkable. Conception occurs when a sperm cell and egg cell join. These cells grow and multiply immensely during the first few weeks of the prenatal period. By the end of the first trimester, key organs have begun to develop, including the brain, heart, and lungs.

The second portion of the prenatal period is referred to as the second trimester. The risk of miscarriage is greatly reduced by this point, and the uncomfortable symptoms of morning sickness generally fade away. At this point, the new mother may experience the urge to urinate more frequently because of pressure caused by the growing fetus. The pregnancy tends to become noticeable to others at this point, and maternity clothes are typically needed now. All of the important organs have already formed in the fetus, and this trimester focuses on further growth and development of these vital body systems.

The final phase of the prenatal period is the third trimester. The new mother may start to feel anxiety concerning the arrival of the baby or the impending process of labor. Leg cramps and mild contractions are common at this time. Heartburn and back pain may be present as well. The doctor will begin to discuss birthing options with the new mom at this stage of pregnancy.

The body systems of the fetus continue to mature in the third trimester. Early labor is possible at this point, and the chances of infant survival is high. Lung development increases dramatically during this time, and most body hair has disappeared.

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Discussion Comments
By ElizaBennett — On May 26, 2011

@SailorJerry - First of all, congratulations to you and your wife, and good luck!

Medically, pregnancy is measured in weeks rather than months. The confusing thing is that they count not from conception, but from the first day of the last menstrual period! That's probably why it seems like your wife's doc is using weird dates. The day your baby was conceived, your wife was already two weeks pregnant!

The trimesters aren't absolute, but each is about 13 weeks. Your wife is in her second trimester and will be until week 27 or so.

As for keeping track, why not sign up for a "pregnancy week by week" e-mail rom a site like BabyCenter or What to Expect? Your wife may already be getting one, but she'll be impressed if you always know how many weeks she is and how your baby is growing.

By SailorJerry — On May 25, 2011

My wife is pregnant with our first baby. She conceived four months ago. What trimester are we in? When do the trimesters begin? And how can I keep track of where she is? The doctor keeps talking about weeks, but I feel like he's a few weeks ahead of when I know we conceived. Help a newbie out!

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