First trimester pregnancy refers to the first three months of pregnancy.

Pregnancy is divided into three trimesters: first, second and third. Each trimester includes three months.

The first trimester pregnancy is most critical period of pregnancy, as most of the body changes and fetal developments happen in this trimester.

In the first month of this trimester, implantation of the fertilized egg in the uterus takes place and the fertilized egg then starts growing and developing in the uterus until delivery.

This process of Implantation generally happens 5-7 days after fertilization. In this stage, the fertilized egg in the uterus is called an embryo.

Shortly after implantation, the corpus luteum produces progesterone, which helps in maintaining the thickened endometrium.

Even the yolk sac starts developing along with the amniotic sac, a fluid-filled sac that protects the baby in the uterus. The yolk sac helps maintain the pregnancy until the placenta and umbilical cord is formed completely.

The placenta and umbilical cord helps carry food and oxygen to the baby from mother’s body and also excretes the wastes from the baby.

During the third week of first trimester pregnancy, the embryo undergoes a developmental process called gastrulation, where the embryo forms multi-layered (the ectoderm, the mesoderm, and the endoderm).

The outer layer called the ectoderm consists of the nerves, skin, and brain. The middle layer called the mesoderm develops into the muscles, heart, sex organs, bones, and blood vessels. The inner layer called the endoderm will have the liver, lungs, intestines, stomach, and urinary tract.

After this process, the neurulation process occurs where the embryo develops the nervous system (entirely). By the end of first month, the head and trunk of the body forms in the embryo.

The eyes, limbs, heart will begin to form within 16-17 days after fertilization. The pumping of fluids through blood vessels starts by 20 days and formation of red blood cells occur by day 21 after conception.

By the end of this month, the size of the embryo is about 0.5 inch and weighs fraction of an ounce.

In the second month of first trimester pregnancy, the development of the baby will be very rapid. Also, major developmental changes occur. The heart of the embryo begins to beat.

All major organs of the body (brain, lungs, liver, pancreas and stomach) begin to appear. The first bone cells, limbs and its feature such as the fingers, toes, and joints starts appear. The formation of inner ear begins at the end of this month.

Now the embryo is called a fetus. Now, the size of the fetus will be one inch and the weight becomes one ounce.

In the third month of first trimester pregnancy, heart chambers will form and the rate of heartbeat increases. The functioning of kidneys and the development of intestines starts.

The external genitalia begins to differentiate, head becomes large; external ears form and the fingers and toes will separate.

By the end of the first trimester pregnancy, the fetus formation completes and is about four inches long and weighs more than an ounce. You can notice all these changes in the first trimester pregnancy.