**Fertilization** is the process in which the egg cell and the sperm unite. This can happen during sexual intercourse in which the sperm are released into the vagina during ejaculation. The sperm swim actively into the Fallopian tube, and a successful sperm will penetrate the egg. After penetration, the nucleus and chromosomes of the sperm cell will unite with that of the egg cell, hence finishing the fertilization process. The fertilized egg now called **zygote** contains a set of chromosomes from each parent

**Placenta** is an organ that is formed during pregnancy and functions to provide oxygen and nutrients to the developing baby. It also removes waste products from the blood of the baby.

The **zygote** will begin to develop and undergo mitosis (cell division) as it travels from the Fallopian tube to the uterine wall, where it attaches itself and continue to develop. The period that caters to this maturation is called **early development**.

The early development stage is composed of three phases called *implantation, gastrulation,* and *neurulation*.

1. **Implantation** – is the phase in which the fertilized egg implants itself in the uterine wall, six or seven days after fertilization. In this phase, the differentiation process starts to occur. Differentiation is the process in which the cells begin to develop specialized tissues for the development of the body.

2. **Gastrulation** – it is when the sorting of cells begins to occur. The blastocyst gradually and periodically sorts itself into two layers, which will then give rise to a third layer. These three layers will eventually be called the **ectoderm, mesoderm**, and **endoderm**.

3. **Neurulation** – it is the phase right after gastrulation, in which the nervous system of the embryo begins to develop. A part of a mesoderm will begin to differentiate into a *notochord*, and then form the neural tube which will eventually mature and develop into an individual’s spinal cord and brain.

Here are the three germ layers:

1. Endoderm – develops into the inner lining of the organs

2. Mesoderm – develops into the tissues and organs

3. Ectoderm – develops into the skin and nervous system

**Childbirth** is the end stage of pregnancy in which the baby exit from the mother’s internal environment via the process of delivery. There are various factors that affect childbirth and delivery including the production and release of specific hormones.

The hormone **oxytocin** plays an important role in childbirth. It is a hormone released from the pituitary gland, and it functions to stimulate a series of contractions in the involuntary muscles of the uterine wall. These contractions will be more frequent and powerful as it forcibly pushes the baby forward and out in the opening of the cervix through the vagina.

Any food or substances that are consumed by a pregnant mother is absorbed by the developing offspring. Certain substances found in medicines and alcohol are not safe for a developing fetus since it can cause heart defects, malformation of body parts, delayed growth, or poor motor development. In this case, a pregnant woman who takes medications and drinks alcohol develops a risk of having a child with birth defects.