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Human Gestation: Definition, Stages & Length

Instructor: Wendy McDougal

Wendy has taught high school Biology and has a master's degree in education.

Human gestation is the growth and development of a human baby in the uterus. This time period is about nine months long. Learn more about the stages of human gestation and take a short quiz at the end.

What is Human Gestation?

There are over seven billion people inhabiting the planet Earth. No two people are exactly alike, and the differences are too vast to count. However, there is one indisputable fact that bonds us all: every human being grew and developed inside a woman's body. This time we spent ''cooking'' in the womb is called gestation. You may know it as pregnancy.

Gestation, or Pregnancy
Pregnant woman

Gestation lasts approximately 40 weeks, or roughly nine months. However, some women may claim that it seemed more like 11 months! Of course, the amount of time does actually vary, from days to weeks. Gestation is divided into three main stages, or trimesters. Each trimester is about three months long. In the following sections, we will look at the gestational period of a human and understand the development that occurs in the various stages.

The First Trimester

You may remember the fifth grade ''talk'' about the birds and the bees. The thought of a tiny creature with a tail swimming into a woman and nine months later a baby appearing seemed absurd, but it's true, and this is where we begin the story. A human being originates when the sperm from a man wins the big race and becomes first to burrow itself into a woman's egg. This miraculous moment is known as conception.

The fertilized egg is now called a zygote. The zygote heads for the uterus and happily nestles into the uterine wall, where it will reside for the next nine months. Thus begins the most dramatic series of transformations of this tiny speck. It will start undergoing many cell divisions in its initial growth. By week five, organogenesis begins. Organogenesis is the development of the brain, heart, and other vital organs. What was once a zygote is now called an embryo.

Fetus at Eight Weeks
Eight week Fetus

By week 12, things have progressed at an astonishing rate. The heart is beating, and all organs are formed. There are stubby little arms and legs present, and there are spots for eyes. Although fairly rudimentary and large-headed, this bun-in-the-oven is now called a fetus and resembles a tiny human being. The fetus weighs in at only about half an ounce, making it entirely possible that the mother doesn't even look pregnant yet.

The Second Trimester

Remember when I said the mother may not even look pregnant by the end of the first trimester? Here's where that dramatically changes. The second trimester, or roughly weeks 13 through 27, marks the decided growth of that tiny human as well as its continued development. If gestation were a drawing, then the first trimester would be the general outline. In the second trimester, detail is added, and shapes are enhanced.

During this time, the fetus continues to put on weight. The mother has most certainly popped out of her regular clothes because of her growing waistline. Such important components like red blood cells and bones are forming. The fetus develops a layer of fat, providing nourishment as well as future chubby cheeks. Adding to the finer details of development, a layer of hair has formed. Fingerprints and footprints are now present.

Most importantly, the immature lungs are now producing surfactant, a substance allowing them to inflate. It is the hope of parents and doctors alike that a fetus makes it to the 27-week mark in utero. Although still extremely premature, a baby born at this point at least has a chance of survival, in part due to the fact that it has lung surfactant.

There are many exciting aspects of the second trimester. Functioning muscles make movement of the fetus apparent, and everyone can feel the baby kick. This also marks the end of suspense for many parents-to-be. At about 20 weeks (and sometimes earlier), parents can view an ultrasound image to see whether their baby is a boy or a girl.

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