Prenatal or antenatal development is the process in which a human embryo or foetus gestates during pregnancy, from fertilization until birth. Often, the terms fetal development, foetal development, or embryology are used in a similar sense.
After fertilization, the process of embryogenesis, (the early stages of prenatal development) begins. By the end of the tenth week of gestational age the embryo has acquired its basic form and the next period is that of fetal development where the organs become fully developed.
This fetal period is described both topically (by organ) and chrononlogically (by time) with major occurrences being listed by gestational age.
Stages during pregnancy. Embryogenesis is marked in green. Weeks and months are numbered
After fertilization, the process of embryogenesis, (the early stages of prenatal development) begins. By the end of the tenth week of gestational age the embryo has acquired its basic form and the next period is that of fetal development where the organs become fully developed.
This fetal period is described both topically (by organ) and chrononlogically (by time) with major occurrences being listed by gestational age.
Definitions of periods
- The perinatal period (from Greek peri, "about, around" and Latin nasci "to be born") 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 (154 days) of gestation (the time when birth weight is normally 500 g) to 7 completed days after birth. In many developing countries, the starting point of this period is considered 28 completed weeks of gestation (or weight more than 1000 g). In ICD-10, a medical classification list by the WHO, there is a particular chapter relating to certain conditions originating in the perinatal period.
- The antepartum period (from Latin ante "before" and parere "to give birth") is literally equivalent to prenatal (from Latin pre- "before" and nasci "to be born"). Practically, however, antepartum usually refers to the period between the 24th/26th week of gestational age until birth, for example in antepartum hemorrhage.
Fertilization
When semen is released into the vagina, the spermatozoa travel through the cervix and body of the uterus and into the Fallopian tubes.
A sperm fertilizing an ovum.
Fertilization of the egg cell (ovum), usually takes place in one of the Fallopian tubes. Many sperm are released with the possibility of just one sperm cell managing to adhere to and enter the thick protective shell-like layer surrounding the ovum.
The first sperm that penetrates fully into the egg donates its genetic material (DNA). The egg then polarizes, repelling any additional sperm.
The resulting combination is called a zygote, a new and genetically unique organism. 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.
Prior to fertilization, each ovum, as gamete contains half of the genetic material that will fuse with the male gamete, which carries the other half of the genetic material (DNA).
The ovum only carries the X female sex chromosome whilst the sperm carries a single sex chromosome of either an X or a male Y chromosome.
The resulting human zygote is similar to the majority of somatic cells because it contains two copies of the genome in a diploid set of chromosomes. One set of chromosomes came from the nucleus of the ovum and the second set from the nucleus of the sperm.
The zygote is male if the egg is fertilized by a sperm that carries a Y chromosome, and it is female if the egg is fertilized by a sperm that 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 genetic information of the zygote comes entirely from the mother via the ovum.
Embryonic period
The embryonic period in humans begins at fertilization (penetration of the egg by the sperm) and continues until the end of the 10th week of gestation (8th week by embryonic age).
The period of two weeks from fertilization is also referred to as the germinal stage.
The embryo spends the next few days traveling down the Fallopian tube.
It starts out as a single cell zygote and then divides 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. It is here that lysis of the zona pellucida occurs.
This process is analogous to zona hatching, a term that refers to the emergence of the blastocyst from the zona pellucida, when incubated in vitro. This allows the trophectoderm cells of the blastocyst to come into contact with, and adhere to, the endometrial cells of the uterus.
The trophectoderm 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 external features begin to take form. This process is called differentiation, which produces the varied cell types (such as blood cells, kidney cells, and nerve cells).>>>>>>>>>>>>>>continue reading
