Fertilization and Early Pregnancy Stages

Fertilization Process

Ovulation occurs about fourteen days before menstruation. The egg released from the Graafian follicle is propelled along the fallopian tube to the uterus, taking over a week for this journey.

The egg can be fertilized for only 24 hours after ovulation while traveling along the fallopian tube.

Intercourse, or the sexual act, is the introduction of the penis into the vagina. At ejaculation, sperm are released inside the vagina.

Unlike the egg, sperm are very fast. They ascend through the vagina, traverse the cervix and uterus, and reach both fallopian tubes in a few hours. They can also survive for three to five days, so if intercourse occurs in the days before ovulation, the probability of pregnancy is very high.

If the egg and sperm meet, sperm surround the egg, but only one penetrates the protective outer layer and enters, leaving its tail outside.

The genetic material of sperm and egg fuse and complete the process of fertilization. The resulting cell is called a zygote.

The Start of Pregnancy

Upon formation of the zygote, a substance is produced that prevents the corpus luteum from degenerating. This allows the corpus luteum to continue producing progesterone, which in turn keeps the endometrium alive and functional.

Because the endometrium does not degenerate, menstruation does not occur, which is usually the first symptom of pregnancy.

The zygote begins to divide very quickly, forming two identical cells, which continue doubling their number every few hours. During the early days of the journey to the uterus, it becomes a few hundred cells that form a hollow sphere.

Once in the uterus, around seven days after fertilization, implantation occurs: the embryo attaches to the endometrium, and the placenta and membranes that surround the fetus begin to form.

The Placenta

The placenta is an organ attached to the uterus, consisting of tissues from both the mother and the fetus. Through the placenta, the fetus receives nutrients and oxygen from the mother and sends waste products and CO2 back. This exchange occurs through blood vessels in the umbilical cord.

In addition, the placenta acts as a filter, preventing harmful microorganisms (viruses and bacteria) from reaching the fetus.

Fetal Membranes

In the uterus, the fetus is surrounded by two membranes: the chorion and the amnion.

• The chorion is the outermost membrane that attaches to the uterus and contributes to forming the placenta.
• The amnion contains the fetus, which is immersed in amniotic fluid. This fluid protects the fetus from shock and pressure.