Embryonic and fetal development

• Embryonic development occurs when the embryo forms and develops.
• It begins with the fertilization of an egg, creating a zygote which then undergoes rapid divisions.
• The scientific term for this process is embryogenesis.
• Stages of pregnancy and development include fertilization, embryonic development via cleavage, fetal development including differentiation, growth and overall development, and ultimately, childbirth.

Fertilization

• The egg remains viable for 12 to 24 hours post-ovulation.
• Sperm are viable for 12 to 48 hours after ejaculation.
• Sperm cells must travel to the uterine tube to enable fertilization.
• Fertilization is when the sperm and egg unite, forming a zygote, the first cell of a new individual.
• The zygote is the result of DNA fusion from sperm and egg.
• Usually occurs in the fallopian tubes.
• The zygote initiates rapid mitotic cell division, marking the beginning of human development.
• About 30 hours post-conception, the zygote, no larger than the head of a pin, continues to divide rapidly.
• It floats down the fallopian tube towards the uterus.

Cleavage

• Cleavage refers to the mitotic division of cells in the early embryo.
• Approximately 96 hours after fertilization, the morula, a solid ball of 32 cells resembling a raspberry, forms.
• A blastula forms about five days after fertilization.
• It is characterized as a ball of over 64 cells surrounding a fluid-filled cavity known as the blastocele.
• The blastula is a product of repeated mitotic division of the zygote.
• Implantation involves the fastening of the embryo to the uterine wall.
• Implantation occurs seven days after fertilization and takes place in the uterus.

The Embryo

• The embryo is the developmental stage from the onset of cleavage until the ninth week.
• The embryo undergoes initial division without growth.
• The embryo enters the uterus at the 16-cell stage.
• It floats freely in the uterus temporarily and relies on uterine secretions for nourishment.

Differentiation and Gastrulation

• Gastrulation involves cell migrations that form the three primary cell layers.
• The ectoderm forms the outer layer, which becomes skin, hair, sweat glands, epithelium, and the brain and nervous system.
• The endoderm forms the inner layer and gives rise to the digestive, respiratory systems, liver, pancreas, bladder, and endocrine glands like the thyroid and parathyroid glands.
• The mesoderm forms the middle layer, developing into body muscles, cartilage, bone, blood, reproductive system organs, and kidneys.

Development through the Third Week

• By 20 days post-fertilization, the embryo measures 0.5 cms and begins organ formation during the third week.
• It is difficult to distinguish between a human and other vertebrate embryo at this stage.

1 Month Development

• At one month, the embryo measures 0.6 cm, with ears, nose, and eyes not yet visible.
• Small arm and leg buds and the backbone can be seen, and heartbeats are detectable.

2 Months Development

• By the second month, when the fetus measures 3 cms, major organ systems form and limb buds develop.
• Limbs become distinct with fingers and toes, bone formation begins, and the eyes move farther apart.
• Around the seventh week, the embryo transitions into a fetus.
• Organ systems forms by the end of the 8th week
• Specialization and organ activities are defined in the fetus.
• Tremendous growth and change in appearances at this stage

Sex Development

• Around the eighth week, the sexually neutral fetus activates gene pathways for sex determination.
• Testes form in XY fetuses, and ovaries form in XX fetuses, while external genitalia develop.

4 Months Development

• At four months, the fetus measures 18 cms, the head enlarges, and facial features become human-like.
• The fetus grows in size, bony parts of the skeleton begin to form, and the mother can feel fetal movements.

7 Months Development

• During this trimester the fetus increases in size.
• At seven months, the fetus is about 40 cms, and the circulatory and respiratory systems mature to prepare for air breathing.
• Fetal growth utilizes much of the mother's protein and calcium intake.
• Maternal antibodies pass to the fetus, conferring temporary immunity during the last month.

9 Months Development

• At nine months, the fetus is about 50 cms, has fat under the skin, resulting in smooth wrinkles.
• Nails are completely formed on fingers and toes, and the baby is at term.

Childbirth Stages

• Labor is the series of events that expel the infant from the uterus.
• Labor initiates with rising estrogen levels and the beginning of uterine contractions.
• The placenta releases prostaglandins, and the pituitary gland releases oxytocin, which combine to produce contractions.

Developmental Aspects of the Reproductive System

• Gender is determined at fertilization.
• Males have XY sex chromosomes, and females have XX sex chromosomes.
• Gonads start forming around the eighth week.

First Trimester

• The first trimester involves cell division and zygote development.
• The developing embryo exhibits a round head, closed eyes, forming taste buds, internal organs moving into the body cavity, and forming upper and lower limbs.

Second Trimester

• During the second trimester, the baby becomes more developed.
• The baby's eyes are open, the sex is determined, the baby weighs 12.5 ounces, and bones and muscles grow steadily at the end.

Third Trimester

• During the third trimester, the baby continues to develop.
• Footprints and fingerprints form, eyelids open and close, and brain development accelerates.
• Followed by the baby being born.