Human fetal development stages Week to Week

The Normal period of gestation or pregnancy is around 9 months and 40 weeks.

Human fetal development stages

First Trimester – It is from conception to 12 weeks. It is the first three months of pregnancy. During this period, the baby will change from a group of cells to a fetus and starts to have a baby’s characteristics.

Week 1 to 4 – During this period, the fertilized egg grows. These eggs would be filled with fluid and a watertight sac surrounds them. This sac is called Amniotic Sac and its purpose is to provide shock absorption to the developing embryo.

During this week, the Placenta develops, an early face forms with dark circles for the eyes, the blood cells forms and begin their circulation, the heartbeats 65 per minute and also mouth, and throat develops.

By the end of the first month, the baby’s size would be 1/4 inch.

Week 5 to 8 – During this period, the ear begins to take shape, tiny buds grow to arms and legs, and also form the toes and fingers.

The eyes also form. The nervous system consists of the brain, spinal cord and other neural tissue are well grown, also the digestive system and sensory organs are formed.

Then the cartilage gets replaced by bones. After the 8th week, this embryo is called a Fetus. It is usually 1 inch long and weighs 0.00094 kg.

Fertilization of Embryo

implantation of embryo -Initial Stages of Human Embryo Development
Initial Stages of Human Embryo Development. Picture By Zephyris – SVG version of ., CC BY-SA 3.0,

The process of fusion of male and female gametes to form a diploid zygote is known as fertilization. The two types of fertilization are external fertilization and internal fertilization. The fusion of gametes that occurs outside the body of the animal is called external fertilization.

An example of external fertilization for frogs is echinoderms. The fusion of the gametes occurs within the body of the female known as internal fertilization. Examples of internal fertilization are reptiles, birds and mammals.

At the time of ovulation, the ovum is fully matured and enters the uterine tube called the infundibulum. Fertilization of the ovum occurs in the ampulla of the uterine tube. Only one spermatozoan pierces the membrane called zona pellucida. Polygamy, the entry of more sperms is prevented by the fertilization membrane around the ovum.

Human fetal development stages: Development of embryo

human fetal development stages

The fertilized ovum is called the zygote. The first phase of embryo development is called cleavage. As a result of cleavage, a ball of cells is formed known as a blastula. The outer surface forms the trophoblast and the embryo gets attached to the uterus wall, which is known as implantation.

The implanted embryo develops extra-embryonic membranes such as amnion, Allantois, Ohorion and yolk sac. The fluid medium of the developing embryo prevents desiccation called the amnion. The fusion of chorion and Allantois is called Placenta.

The exchange of gases between the mother and foetus and also their elimination of nitrogenous waste from the foetus are functions of the placenta. The embryo and the placenta are connected by an umbilical cord which is derived from the Allantois.

From the fertilization of the ovum to the birth of the baby of about 9 months during which the foetus remains inside the uterus is called the re-gestation period. the development of the foetus can be studied from the first, second and third trimesters.

The proliferation of cells takes place and gradually a single cell is transformed into the foetus is called the first trimester. Organogenesis takes place in the first trimester. Organogenesis means the formation of organs.

The foetus grows rapidly in the second trimester. The respiratory and circulatory systems become well developed and functional in the second trimester. The length and weight of the foetus increase very rapidly and development is completed in the third trimester.

The uterus begins to contract rhythmically under the influence of the oxytocin hormone. The contractions become stronger and more frequent called labour pain. With continued powerful contractions, the amnion ruptures and the fluid that flows out through the Female Reproductive System is amniotic.

The placenta breaks away from the uterus and is expelled after birth. The first milk that comes out from the mother’s mammary gland just after childbirth is Colestrum. Rich in proteins and nutrients and also containing antibodies to provide immunity for the newborn infant is Colestrum.

The secretion of the milk is stimulated by the pituitary hormone called prolactin. The best milk easily available clean, uncontaminated and sterile is mother’s milk. The milk contains antibodies that shield the body from external viral and bacterial infections in mother’s milk.

In rural areas, breast milk is used as eye drops for viral conjunctivitis. The calorific value of breastmilk is 70 per 100 ml of milk. The protein in breast milk that provides protection against intestinal and respiratory infections is lactoferrin.

The technique of in-vitro fertilization, fertilization and early development takes place in an artificial medium outside called test-tube babies. The invention of first test-tube baby is Steptoe and Edwards in the UK.

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* * All the Notes in this blog, are referred from Tamil Nadu State Board Books and Samacheer Kalvi Books. Kindly check with the original Tamil Nadu state board books and Ncert Books.
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