Pregnancy and Embryonic Development

The journey from fertilisation to the birth of a fully formed baby is called pregnancy, and the process by which the baby grows inside the mother is embryonic development. This section traces that entire timeline, from the moment a zygote is formed to the moment a child is delivered.

From Zygote to Blastocyst

After fertilisation in the ampulla of the fallopian tube, the single-celled zygote immediately begins to divide by mitosis — a process called cleavage. These divisions are rapid, and the cells (now called blastomeres) get smaller with each division without increasing the overall size of the embryo. The embryo moves down the fallopian tube toward the uterus as it divides.

By the time it reaches the 8- to 16-cell stage, the embryo is called a morula (from the Latin for mulberry, which it resembles). The morula continues to divide and, as it enters the uterine cavity, it transforms into a blastocyst. A blastocyst has two distinct parts:

• An outer layer of cells called the trophoblast.
• An inner group of cells called the inner cell mass.

The trophoblast will later form the placenta, while the inner cell mass will develop into the actual embryo.

Implantation

Once the blastocyst reaches the uterus, it needs to attach itself to the uterine lining to continue growing. This attachment is called implantation. The trophoblast cells secrete enzymes that digest the endometrial tissue, allowing the blastocyst to embed itself into the endometrium. Implantation is completed by about the 7th day after fertilisation.

After implantation, the uterine endometrium undergoes further changes and is now called the decidua. The trophoblast cells also begin to develop finger-like projections called chorionic villi, which grow into the endometrium. Together, the chorionic villi and the surrounding uterine tissue form the placenta.

The Placenta: A Lifeline

The placenta is a temporary organ that connects the developing embryo to the mother's uterine wall. It serves as the interface for the exchange of nutrients, oxygen, and waste products between the mother and the foetus. The placenta performs several critical functions:

• Supply: It delivers oxygen and nutrients (like glucose, amino acids, and vitamins) from the mother's blood to the foetus.
• Removal: It carries away carbon dioxide and nitrogenous wastes (like urea) from the foetal blood to the mother's blood for excretion.
• Barrier: It prevents many harmful substances and pathogens from reaching the foetus, though it is not a perfect barrier (some viruses and drugs can cross it).
• Hormone production: The placenta acts as an endocrine gland. It secretes several hormones essential for maintaining pregnancy, including:
  • Human chorionic gonadotropin (hCG): This hormone is produced by the trophoblast cells soon after implantation. It maintains the corpus luteum in the ovary, which continues to secrete progesterone and estrogen to keep the uterine lining intact. hCG is the hormone detected in pregnancy tests.
  • Human placental lactogen (hPL): Also called human chorionic somatomammotropin, this hormone helps regulate maternal metabolism and promotes the growth of the mammary glands.
  • Estrogens and Progesterone: These are produced in large quantities by the placenta after the first trimester. They are crucial for maintaining the pregnancy, suppressing ovulation, and preparing the breasts for lactation.

The foetus is connected to the placenta by the umbilical cord, which contains two umbilical arteries (carrying deoxygenated blood and wastes from the foetus to the placenta) and one umbilical vein (carrying oxygenated blood and nutrients from the placenta to the foetus).

The Three Trimesters of Pregnancy

Human pregnancy lasts about nine months (approximately 40 weeks from the last menstrual period) and is divided into three trimesters of three months each.

Parturition and Lactation …