The first two weeks of the human development are called the preembryonic period. This period begins with the fertilization. Fertilization is the beginning of the pregnancy and can be considered as the beginning of a new life.
In order to have successful fertilization, several events have to be take their course (e.g. gametogenesis). Female gamete (oocyte) is produced during the menstrual cycle and expelled during the ovulation. During each ovarian cycle, only one follicle with an oocyte reaches full maturity. At the 14th day in an average 28-day cycle this follicle bulges on the surface of the ovary. Immediately before the ovulation, the oocyte and some surrounding cells (cumulus oophorus) detach from the interior of the follicle. Also, shortly before the ovulation the fimbriae of the oviduct start covering the surface of the ovary. During ovulation, follicle bursts and the oocyte is expelled into the uterine tube.

1. Corona radiata
2. Spindle
3. Zona pellucida
4. Sperm

The expelled secondary oocyte is surrounded by the zona pellucida and several layers of the follicular cells arranged as the corrona radiata.
Male gametes are produced during the spermatogenesis and stored in the epididymis. Upon ejaculation into the female genital tract, the spermatozoa are not capable of fertilizing the oocyte. They must undergo a capacitation period that lasts approximately 7 hours, during which the glycoprotein coat and seminal proteins are removed from the surface of the sperm acrosome by the action of the substances secreted by uterus or uterine tubes. When capacitated spermatozoa come into contact with the corrona radiata surrounding the secondary oocyte, they undergo the acrosomal reaction. This process includes release of the acrosomal vesicles content that helps the sperm digest its way to the oocyte plasma membrane in order to fuse with it.

Fertilization, the process by which the male and female gametes fuse, marks the beginning of the pregnancy. It lasts 24 hours and occurs in the ampullary region of the uterine tube. The first event is the scattering of the corrona radiata cells by the released contents of the acrosomal vesicle (hyaluronidase), tubal mucosa enzymes and sperm tail movements. Penetration of the zona pellucida is enabled by the action of other enzymes released from the acrosome (acrosin and neuraminidase). When the first sperm passes through the zona pellucida, cortical and zona reaction make it impermeable to other sperms. This mechanism ensures that each oocyte is fertilized by only one sperm.

When the sperm enters the oocyte, it leaves its plasma membrane behind. After the sperm entry, the secondary oocyte finishes its second meiotic division, forming an ovum and a second polar body. The nucleus of the mature oocyte is known as the female pronucleus. Male pronucleus is formed by the enlarging of the nucleus in the head of the sperm. During the growth of the pronuclei they replicate their DNA. At this stage, male and female pronuclei are indistinguishable. Fertilization ends with the fusion of female and male pronucleus and formation of the zygote. Within 24-48 hours after fertilization, early pregnancy factor (EPF) can be detected in the maternal serum.