Prenatal development


Prenatal development involves the development of the embryo and of the fetus during a viviparous animal's gestation. Prenatal development starts with fertilization, in the germinal stage of embryonic development, and continues in fetal development until birth. The term "prenate" is used to describe an unborn offspring at any stage of gestation.
In human pregnancy, prenatal development is also called antenatal development. The development of the human embryo follows fertilization, and continues as fetal development. By the end of the tenth week of gestational age, the embryo has acquired its basic form and is referred to as a fetus. The next period is that of fetal development where many organs become fully developed. This fetal period is described both topically and chronologically with major occurrences being listed by gestational age.
The very early stages of embryonic development are the same in all mammals, but later stages of development, and the length of gestation varies.

Terminology

In the human:
Image:Prenatal development table.svg|center|thumb|900px|Stages during pregnancy. Embryonic development is marked in green. Weeks and months are numbered by gestation.
Different terms are used to describe prenatal development, meaning development before birth. A term with the same meaning is the "antepartum" Sometimes "antepartum" is however used to denote the period between the 24th/26th week of gestational age until birth, for example in antepartum hemorrhage.
The perinatal period is "around the time of birth". In developed countries and at facilities where expert neonatal care is available, it is considered from 22 completed weeks of gestation to 7 completed days after birth. In many of the developing countries the starting point of this period is considered 28 completed weeks of gestation.

Fertilization

Fertilization marks the first germinal stage of embryonic development. When semen is released into the vagina, the spermatozoa travel through the cervix, along the body of the uterus, and into one of the fallopian tubes where fertilization usually takes place in the ampulla. A great many sperm cells are released with the possibility of just one managing to adhere to and enter the thick protective layer surrounding the egg cell. The first sperm cell to successfully penetrate the egg cell donates its genetic material to combine with the DNA of the egg cell resulting in a new one-celled zygote. The term "conception" refers variably to either fertilization or to formation of the conceptus after its implantation in the uterus, and this terminology is controversial.
The zygote will develop into a male if the egg is fertilized by a sperm that carries a Y chromosome, or a female if the sperm carries an X chromosome. The Y chromosome contains a gene, SRY, which will switch on androgen production at a later stage leading to the development of a male body type. In contrast, the mitochondrial DNA of the zygote comes entirely from the egg cell.

Development of the embryo

Following fertilization, the embryonic stage of development continues until the end of the 10th week . The first two weeks from fertilization is also referred to as the germinal stage or preembryonic stage.
The zygote spends the next few days traveling down the fallopian tube dividing several times to form a ball of cells called a morula. Further cellular division is accompanied by the formation of a small cavity between the cells. This stage is called a blastocyst. Up to this point there is no growth in the overall size of the embryo, as it is confined within a glycoprotein shell, known as the zona pellucida. Instead, each division produces successively smaller cells.
The blastocyst reaches the uterus at roughly the fifth day after fertilization. The blastocyst hatches from the zona pellucida allowing the blastocyst's outer cell layer of trophoblasts to come into contact with, and adhere to, the endometrial cells of the uterus. The trophoblasts will eventually give rise to extra-embryonic structures, such as the placenta and the membranes. The embryo becomes embedded in the endometrium in a process called implantation. In most successful pregnancies, the embryo implants 8 to 10 days after ovulation. The embryo, the extra-embryonic membranes, and the placenta are collectively referred to as a conceptus, or the "products of conception".
Rapid growth occurs and the embryo's main features begin to take form. This process is called differentiation, which produces the varied cell types. A spontaneous abortion, or miscarriage, in the first trimester of pregnancy is usually due to major genetic mistakes or abnormalities in the developing embryo. During this critical period the developing embryo is also susceptible to toxic exposures, such as:
The embryo passes through 3 phases of acquisition of nutrition from the mother:
  1. Absorption phase: Zygote is nourished by cellular cytoplasm and secretions in fallopian tubes and uterine cavity.
  2. Histoplasmic transfer: After nidation and before establishment of uteroplacental circulation, embryonic nutrition is derived from decidual cells and maternal blood pools that open up as a result of eroding activity of trophoblasts.
  3. Hematotrophic phase: After third week of gestation, substances are transported passively via intervillous space.

    Development of the fetus

The first ten weeks of gestational age is the period of embryogenesis and together with the first three weeks of prenatal development make up the first trimester of pregnancy.
From the 10th week of gestation, the developing embryo is called a fetus. All major structures are formed by this time, but they continue to grow and develop. Because the precursors of the organs are now formed, the fetus is not as sensitive to damage from environmental exposure as the embryo was. Instead, toxic exposure often causes physiological abnormalities or minor congenital malformation.

Development of organ systems

Development continues throughout the life of the fetus and through into life after birth. Significant changes occur to many systems in the period after birth as they adapt to life outside the uterus.

Fetal blood

first takes place in the yolk sac. The function is transferred to the liver by the 10th week of gestation and to the spleen and bone marrow beyond that. The total blood volume is about 125 ml/kg of fetal body weight near term.

Red blood cells

Megaloblastic red blood cells are produced early in development, which become normoblastic near term. Life span of prenatal RBCs is 80 days. Rh antigen appears at about 40 days of gestation.

White blood cells

The fetus starts producing leukocytes at 2 months gestational age, mainly from the thymus and the spleen. Lymphocytes derived from the thymus are called T lymphocytes, whereas those derived from bone marrow are called B lymphocytes. Both of these populations of lymphocytes have short-lived and long-lived groups. Short-lived T cells usually reside in thymus, bone marrow and spleen; whereas long-lived T cells reside in the blood stream. Plasma cells are derived from B cells and their life in fetal blood is 0.5 to 2 days.

Glands

The thyroid is the first gland to develop in the embryo at the 4th week of gestation. Insulin secretion in the fetus starts around the 12th week of gestation.

Cognitive development

Electrical brain activity is first detected at the end of week 5 of gestation. Synapses do not begin to form until week 17. Neural connections between the sensory cortex and thalamus develop as early as 24 weeks' gestational age, but the first evidence of their function does not occur until around 30 weeks, when minimal consciousness, dreaming, and the ability to feel pain emerges. REM sleep develops at around 30 weeks and comprises the majority of sleep. The proportion of REM sleep is progressively reduced to 58% by 36–38 weeks.
Initial knowledge of the effects of prenatal experience on later neuropsychological development originates from the Dutch Famine Study, which researched the cognitive development of individuals born after the Dutch famine of 1944–45. The first studies focused on the consequences of the famine to cognitive development, including the prevalence of intellectual disability. Such studies predate David Barker's hypothesis about the association between the prenatal environment and the development of chronic conditions later in life. The initial studies found no association between malnourishment and cognitive development, but later studies found associations between malnourishment and increased risk for schizophrenia, antisocial disorders, and affective disorders.
There is evidence that the acquisition of language begins in the prenatal stage. After 26 weeks of gestation, the peripheral auditory system is already fully formed. Also, most low-frequency sounds can reach the fetal inner ear in the womb of mammals. Those low-frequency sounds include pitch, rhythm, and phonetic information related to language. Studies have indicated that fetuses react to and recognize differences between sounds. Such ideas are further reinforced by the fact that newborns present a preference for their mother's voice, present behavioral recognition of stories only heard during gestation, and present preference for their native language. A more recent study with EEG demonstrated different brain activation in newborns hearing their native language compared to when they were presented with a different language, further supporting the idea that language learning starts while in gestation.