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From Embryo To Baby: Human Development

by — Last updated: 2014-05-08 in Pregnancy

Ever wondered how a baby grows and develops inside the womb? It is not a simple process, but is quite amazing. Here is a description of how a baby develops, from the moment of conception, passing through its organs and body formation until its birth.

The human body, an amazing piece of work

Modern medical techniques allow future moms and dads to follow up the development of their baby. The normal ultrasound that can show you how your baby grows inside the belly, to the more recent 3D ultrasound technique, where you can even see if your baby is making faces. But even when pregnancy follow-ups are performed regularly, you can’t really see how your baby’s body is developing day by day.

Human development, and of every other animal, is an amazing process that occurs from the moment the spermatozoid fertilizes the egg. What is the first thing that starts forming? Is it the skeleton? Or is it the heart? In the next lines you will be able to explore the development of a baby, from its conception to its birth, and which main characteristics are formed on each stage of its growth.

The first step: fertilization

During sexual intercourse, semen is released into the uterine cavity of the woman and a race between the spermatozoids present in it to reach an egg begins.

Spermatozoids swim through the uterus until they get to the fallopian tubes, which is the place where an egg would be available for any of them to fertilize.

The journey is not easy and only a few spermatozoids make it until this point. When encountering an egg, spermatozoids try to get pass through its outer layer and usually, just one of them will be able do this. Then both cells combine their information and form what is known as zygote. The process that comprehends the formation of the zygote lasts around 24 hours.

Fertilization of one egg by more than one sperm can occur, but it is pretty uncommon and the result of this would be an unviable zygote. Fertilization of more than one egg can happen too, which gives as a result the formation of two, three or more zygotes. 

The pre-embryonic stage

After fertilization, the newly formed zygote travels from the fallopian tube to the uterus. Meanwhile, it starts a division process, which will give as a product a tiny raspberry shaped body, known as morula.

The morula is conformed by two types of cells: the ones covering it, which will form the placenta, and the inner cells, which will give rise to the embryo. 

See Also: Signs Of Consciousness In Baby Brains From As Early As 5 Months

All these processes happen in a period of five days and in the sixth, the now called blastocyst reaches the uterus and attaches to the uterine wall.

By this stage, the uterine wall is covered by endometrium, a layer of epithelial cells and blood vessels that functions as a cushion for the blastocyst to stick.  When no egg is fertilized, this cushion detaches from the uterus and initiates a process that occurs every 28 days, more or less, in every woman, known as menstruation.

The Embryonic Period

It has been two weeks after fertilization now and the blastocyst is fully implanted on the uterine wall. The embryonic period starts and the formation of the baby’s organs too.  The embryo is still very tiny at this stage, almost as tiny as the tip of a pen.

From cells to a fully formed tiny human

The embryo is formed by three layers of cells, which will give rise to different organs, depending on their localization.

The ectoderm is the outermost layer and it gives rise to the nervous system, the ears, nose and eyes, the skin, hair, nails and teeth, and the pituitary and mammary glands. The mesoderm is made of the cells located in the middle of the embryo. From these the baby’s bones, kidneys, heart and reproductive system will form. Finally, the inner cell layer, also known as endoderm is the one from which the gastrointestinal, respiratory and urinary systems derive, as well as the thyroid and parathyroid glands, the thymus, the pancreas and the liver.

Before week three ends, the central nervous system starts developing, followed by the spinal cord and every other system that will form the embryo’s body. When week four starts, the heart of the embryo is already pumping blood.

Cardiovascular system in general is the first system to start functioning in a baby.

At the same time, the musculoskeletal, digestive, respiratory, urogenital and the rest of the nervous system, as well as the sense organs, begin to develop. Also, the head and neck start forming, giving the embryo its particular C-shaped appearance. As time goes by, facial features and extremities will derive from their correspondent embryonic structures.

Five weeks after conception you can already identify the embryo’s tiny nostrils and in week 8, it’s eyes and nose are already visible, as well as the structures that will form its arms, legs, fingers and toes.

The final countdown

The embryonic period ends by the 8th week, after which the new being is called fetus.

From this time on, the fetus will start growing rapidly and its tissues will mature during this stage.

See Also: IVF And Twins: Stronger Embryo Helps A Weaker One Survive

Would you like to know if it is a boy or a girl? At the 9th week after conception, the fetus genitals will start to develop according to the information carried in the Y chromosome. So, basically the information provided by the father is the one that determines the sex of the baby and you will be able know if it's a boy or a girl by the third month.

There are still 6 more months to go. During this time, the fetus continues its growth and starts moving. By the ninth month, it reaches a size of approx.. 40 cm. and weighs from 6 to 8 pounds. It is ready now to come into the outer world. 

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