Describe the distinct periods in pre-natal development?

There are three distinct periods in prenatal development:

1. Germinal period: This period begins at conception and lasts for about two weeks. During this time, the fertilized egg, or zygote, undergoes rapid cell division and travels down the fallopian tube to implant in the uterus. The zygote forms a blastocyst, which eventually develops into the embryo. The germinal period ends when the embryo is fully implanted in the uterine wall.

2. Embryonic period: This period lasts from the third week to the eighth week of pregnancy. During this time, the embryo undergoes rapid growth and development. The major organs and systems of the body begin to form, including the heart, brain, spinal cord, and limbs. By the end of the embryonic period, the embryo is about the size of a grape and is referred to as a fetus.

3. Fetal period: This period begins at the ninth week of pregnancy and lasts until birth. During this time, the fetus continues to grow and develop, with organs and systems becoming more complex and functional. The fetus also begins to exhibit movement and can respond to external stimuli. By the end of the fetal period, the fetus is fully developed and ready to be born.

Pre-Embryonic Period (0-2 weeks after fertilization)

- Division of the fertilized egg into multiple cells
- Formation of the blastocyst, which implants into the uterine wall
- Differentiation of the three germ layers: ectoderm, mesoderm, and endoderm

Embryonic Period (3-8 weeks after fertilization)

- Rapid organogenesis
- Development of head, limbs, and major organs
- Formation of the placenta
- Fetal heartbeat is detectable
- Neural tube closes

Fetal Period (9 weeks until birth)

- Organs mature and grow
- Hair, nails, and eyelashes develop
- Fetus becomes viable (capable of surviving outside the womb)
- Rapid weight gain
- Brain and nervous system continue to develop
- Respiratory and digestive systems mature

Specific Stages within the Fetal Period:

First Trimester (9-12 weeks):

- Fetus grows from about 1 inch to 3.5 inches
- Major organs are formed
- Sexual organs develop

Second Trimester (13-27 weeks):

- Fetus grows from about 3.5 inches to 14 inches
- Quickening (fetal movement) is felt by the mother
- Lanugo hair (fine hair) covers the body
- Skin wrinkles begin to smooth out

Third Trimester (28 weeks until birth):

- Fetus grows from about 14 inches to 20 inches
- Fat is deposited
- Toenails and fingernails appear
- Fetus assumes a head-down position