Fetus at 12 weeks (source: www.nlm.nih.gov).
National Institutes of Health
When sperm is deposited in the vagina, it travels through the cervix and into the Fallopian tubes. Conception usually takes place in the Fallopian tube. A single sperm penetrates the mother’s egg cell, and the resulting cell is called a zygote.
The zygote contains all of the genetic information (DNA) necessary to become a child. Half of the genetic information comes from the mother’s egg, and half from the father’s sperm.
The zygote spends the next few days traveling down the Fallopian tube and divides to form a ball of cells. Further cell division creates an inner group of cells with an outer shell. This stage is called a “blastocyst”. The inner group of cells will become the embryo, while the outer group of cells will become the membranes that nourish and protect it.
The blastocyst reaches the uterus at roughly the fifth day, and implants into the uterine wall on about day six. At this point in the mother’s menstrual cycle, the endometrium (lining of the uterus) has grown and is ready to support a fetus. The blastocyst adheres tightly to the endometrium, where it receives nourishment via the mother’s bloodstream.
The cells of the embryo now multiply and begin to take on specific functions. This process is called differentiation, which produces the varied cell types that make up a human being (such as blood cells, kidney cells, and nerve cells).
There is rapid growth, and the baby’s main external features begin to take form. It is during this critical period of differentiation (most of the first trimester) that the growing baby is most susceptible to damage from:
- Alcohol, certain prescription and recreational drugs, and other substances that cause birth defects
- Infection (such as rubella or cytomegalovirus)
- Radiation from x-rays or radiation therapy
- Nutritional deficiencies
Specific Changes by Week
Week 3Beginning development of the brain, spinal cord, and heart Beginning development of the gastrointestinal tract
Weeks 4 to 5Formation of tissue that develops into the vertebra and some other bones Further development of the heart which now beats at a regular rhythm Movement of rudimentary blood through the main vessels Beginning of the structures of the eye and ears The brain develops into five areas and some cranial nerves are visible Arm and leg buds are visible
Week 6Beginning of formation of the lungs Further development of the brain Arms and legs have lengthened with foot and hand areas distinguishable Hands and feet have digits, but may still be webbed
Week 7Nipples and hair follicles form Elbows and toes visible All essential organs have at least begun to form
Week 8Rotation of intestines Facial features continue to develop The eyelids are more developed The external features of the ear begin to take their final shape
The end of the eighth week marks the end of the “embryonic period” and the beginning of the “fetal period”.