Pregnancy and Embryonic Development

After fertilisation, the zygote undergoes repeated mitotic divisions to form a blastocyst, which then implants into the endometrium of the uterus - a process called implantation. Once implantation is complete, pregnancy begins. The period from fertilisation to delivery in humans lasts approximately nine (9) months, also called the gestation period.

The human foetus within the uterus

Gastrulation is the early developmental process in which the embryo changes from a single-layered blastula into a three-layered structure called the gastrula.

Placenta is a temporary organ formed during pregnancy that connects the foetus to the uterine wall and allows exchange of nutrients, gases and wastes between the mother and the foetus.

After implantation of the blastocyst into the uterine wall, the outer layer of the embryo (trophoblast) develops finger-like projections called chorionic villi.

• These villi penetrate and become surrounded by uterine tissue and maternal blood.
• The chorionic villi and uterine tissue become interdigitated (interlocked) with each other.
• Together, they form a structural and functional unit called the placenta.

The placenta is a disc-shaped structure connecting the developing embryo (fetus) to the mother's uterine wall.

Connection to Fetus: The placenta is connected to the embryo through the umbilical cord, which contains blood vessels that transport substances between the mother and the embryo.

Functions of the Placenta:

The placenta acts as a temporary endocrine gland and secretes several hormones critical for maintaining pregnancy.

Placental Hormones

Other Hormones Elevated During Pregnancy:

In addition to placental hormones, levels of the following hormones also increase several-fold in maternal blood: - Cortisol - regulates metabolism and immune response - Prolactin - prepares mammary glands for lactation - Thyroxine - supports increased metabolic demand

Purpose of Elevated Hormones:
Increased production of these hormones is essential for: 

1. Supporting fetal growth and development
2. Causing metabolic changes in the mother's body
3. Maintaining pregnancy throughout its duration

Immediately after implantation, the inner cell mass (embryo proper) begins to differentiate into distinct tissue layers.

The Three Primary Germ Layers:

Inner Cell Mass (Embryo)
(Differentiates into)

↓

Ectoderm - Outer germ layer

Endoderm - Inner germ layer

(Later, a third layer develops between them)

↓

Mesoderm - Middle germ layer

These three germ layers give rise to all tissues and organs in the adult human body.

Organs Derived from Each Germ Layer:

The inner cell mass contains a special population of cells called stem cells.

• Stem cells have the ability (potency) to differentiate into any tissue or organ of the body.
• They are described as pluripotent - able to give rise to all cell types.
• Stem cells are of great medical importance in regenerative medicine.

Duration of Human Pregnancy:

The human gestation period (duration of pregnancy) is 9 months, divided into three trimesters.

Month-by-Month Developmental Milestones:

First Trimester

• End of Month 1: The embryo's heart is formed - the first major organ to develop. - The heartbeat can be detected using a stethoscope.
• End of Month 2: The fetus develops limbs and digits (fingers and toes). - At this stage, the embryo is officially referred to as a fetus.
• End of 12 Weeks (End of First Trimester): Most major organ systems are formed. - The limbs are well-developed. - The external genital organs are distinguishable and well-developed.

Second Trimester

• During Month 5 (Around Week 20): The first movements of the fetus are felt by the mother (called quickening). - Hair appears on the head of the fetus.
• End of 24 Weeks (End of Second Trimester): The body is covered with fine hair (lanugo). - Eyelids separate (they were fused earlier in development). - Eyelashes are formed.

Third Trimester

• End of 9 Months (Completion of Pregnancy): The fetus is fully developed with all organs mature and functional. - The fetus is ready for delivery.

• Placenta is formed after implantation and helps in exchange of nutrients, oxygen and wastes between mother and foetus.
• Three germ layers (ectoderm, mesoderm and endoderm) are formed, which develop into all body organs.
• Hormones like hCG, progesterone and estrogen increase to support pregnancy and foetal growth.
• Foetal development occurs gradually, and by nine months the foetus is fully developed and ready for birth.