THE MIRACLES OF CONCEPTION: BACK TO THE BASICS

The process of human reproduction is remarkable in its complexity. Broadly divided into the stages of ovulation, fertilization and implantation, human reproduction embodies multi-levels of challenges and medical wonders.

Ovulation

The human ovary comprises of millions of follicles, hundreds of which will develop during a woman’s lifetime. During each menstrual cycle, the release of follicle stimulating hormone (FSH) triggers the growth and development of several ovarian follicles. Upon stimulation by the luteinizing hormone (LH), one mature follicle releases an egg into the reproductive tract. The egg then enters into the Fallopian tube, and migrates towards the uterine cavity.

At this stage, unless the mature egg is fertilized by sperm within 24 hours, it will lose its viability and perish. Rarely, two eggs may be released during a cycle, and the fertilization of both egg would lead to fraternal twins. Meanwhile, following stimulation by FSH, the uterus also undergoes maturation to develop an internal lining, known as endometrium. The endometrium is nutrient-rich and is essential to support a fertilized egg.

To increase the likelihood of pregnancy, women may time intercourse with their ovulation dates. Fortunately, the release of egg can be predetermined by measuring LH hormone levels in the urine.  Ovulation detection kits are available to women who are trying to determine their most fertile days of the month. In addition, a woman’s cervical secretion changes during ovulation, becoming more slippery in consistency.

Ovulation also leads to a slight fall in body temperature followed by a spike, and this change can be measured using oral or vaginal thermometers. The other common symptoms of ovulation are heightened sex drive, bloating, or light spotting.

Fertilization

When a man ejaculates, his sperm is deposited into the woman’s vagina. The next step of reproduction is dependent on the sperm surviving the hostile environment of the cervix and uterus, and migrating its way up to find the mature egg in the Fallopian tube. This is a long journey for the sperm, and it is estimated that only 1 in 14 million sperm survives to meet an egg.

However, once within the Fallopian tube, sperm can survive the nutrient-rich environment for a few days. Upon meeting an egg, the sperm will have to penetrate the tough outer shell, known as the zona pellucida. Again, the chances of penetrating the zona pellucida are slim, and only the healthiest sperm will make it through.

The next step after entry into the egg, is a cascade of events leading to the arrangement of the egg DNA and sperm DNA into structures known as pronuclei. A microscopic examination of the egg at this stage, would reveal the pronuclei at the center. The egg has now undergone fertilization to form a zygote (early embryo). Once a sperm enters the egg, the entry of another sperm is blocked by changes in the egg’s shell.

Implantation

A zygote will start to replicate and double its genetic material within a day of formation. An exponential event of cell division and growth is set forth, wherein one cell leads to two, two to four, four to eight, and so on. On days 4 – 6, following fertilization, the embryo is between 50 – 100 cells, and enters into the blastocyte stage. The blastocyte is characterized by a hollow structure, the inside cell mass of which represents the growing embryo, and the outer wall represents the early placenta.

After fertilization, the embryo passes down the Fallopian tube, and during the blastocyst (day 5 or 6 embryo) enters into the uterus. The embryo then breaks out of its shell to establish direct contact with the endometrium of the uterus.

Through an exchange of hormones between the embryo and uterus, the embryo will become connected with the mother through blood vessels.  This process is called implantation. Sometimes women experience light spotting during implantation. A mucus plug develops in the woman’s cervix to block the passage.

Implantation represents a giant step in pregnancy. Around 2 weeks from fertilization, a woman’s hormones reach levels that are easily detected by a pregnancy test. The next stages of the pregnancy will see the rapid and dramatic transformation of the embryo to form specialized cells and structures.