Prenatal Stages and Development: Germinal, Embryonic & Fetal Period

  • 0:07 Prenatal Stages of Development
  • 1:20 Germinal Period
  • 2:23 Embryonic Period
  • 4:24 Fetal Period
  • 5:51 Lesson Summary
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Lesson Transcript
Instructor: Lisa Roundy

Lisa has taught at all levels from kindergarten to college and has a master's degree in human relations.

In the 9 months before a baby is born, there is an enormous amount of prenatal development. In this lesson, you will explore the following stages of prenatal development: the germinal period, the embryonic period, and the fetal period.

Prenatal Stages of Development

'Steve, it's time!'

Jennifer is letting her husband know that the moment they've been planning for is here, and Steve rushes his wife to the hospital. They've been waiting for the birth of their baby to take place and are very excited. Steve and Jennifer have been preparing for this moment by taking infant care classes, preparing a nursery, and buying lots of diapers. However, they're not the only ones who have been getting ready for this day. The baby has spent the past 9 months inside its mother, preparing as well.

Prenatal development is the process by which a baby develops inside the mother's womb. Prenatal development takes about 38 weeks to complete. During this time, a single cell becomes a full-term baby.

This time period is divided into different categories of development. The 3 primary stages of prenatal development are the germinal period, the embryonic period, and the fetal period. Each of these stages is important for reaching the ultimate goal of a healthy baby.

Now, let's look at how Steve and Jennifer's baby has been preparing for birth over the last 9 months.

Germinal Period

Jennifer's baby began to develop at the moment of conception; this is when the mother's egg is fertilized by the father's sperm. Pregnancy is considered to begin at first day of the mother's last menstrual period. Conception typically occurs at what is considered the 2nd week of pregnancy.

Conception is also the beginning of the germinal period of prenatal development. This stage occurs during the first 2 weeks of development (or the 2nd - 4th weeks of pregnancy).

The union of the sperm and egg cell forms a single cell called a zygote. The zygote moves along the Fallopian tube toward the uterus. At the same time, the zygote is rapidly dividing in a process called cleavage.

Once it reaches the uterus, the zygote becomes what is called a blastocyst, and it begins to implant into the wall of the uterus. The germinal period ends when the blastocyst is fully implanted into uterine tissue.

Embryonic Period

Think of Steve and Jennifer's baby as spending the germinal period looking for a place to live. Now that she's found her place, she's ready to start organizing for the work she has ahead of her.

This is the embryonic period of prenatal development. This stage lasts from implantation until about 8 weeks from the time of conception (or the 10th week of pregnancy). This is the most important time of prenatal development because the embryo is developing the foundations for a healthy baby.

The blastocyst that implanted in the uterus continues to divide rapidly after implantation. Through a process called differentiation, cells begin to take on different functions. One of the first examples of this is a division between the cells that will make up the placenta and the cells that will make up the baby.

After this, a process called gastrulation forms three layers called germ layers. The outer layer is called the ectoderm, the middle layer is called the mesoderm, and the inner layer is called the endoderm.

Each germ layer will differentiate into different structures. The ectoderm will form many outer tissues such as skin and hair, as well as most of the nervous system tissues - including the brain. The mesoderm will form tissues inside the body such as the lungs, bones, and muscles - including the heart. The endoderm will form tissues such as the digestive tract and bladder, as well as other internal organs.

All of the essential structures have been formed (both inside and outside) by the time the embryonic period comes to an end. The new title of fetus is now given to the embryo.

Embryogenesis is a term that can be used to describe all of the processes of embryo development up until the time it becomes a fetus.

Fetal Period

Remember Steve and Jennifer's baby? We said that the germinal period was like their baby finding a place to live, and that during the embryonic period, she was organizing her space. During the final period of development, she is putting on the finishing touches in preparation of the big event.

This final stage of development is called the fetal period of development. This stage takes place from the end of the embryonic period until the time of birth (or the 10th to 40th weeks of pregnancy).

The most notable development during this stage is the dramatic change in size that takes place. When the baby becomes a fetus, it is less than an inch and a half long and weighs less than 1/10 of an ounce. By the time the baby is considered full term at 38 weeks, it will probably be over 20 inches long and weigh about 7 1/2 pounds.

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