Periods of Ontogenesis
Ontogenesis is the complete cycle of an organism's development from zygote formation to death. It is divided into three stages:
- Prezygotic period. Involves the formation of germ cells (gametes) with a halved (haploid) set of chromosomes via meiosis.
- Prenatal (antenatal) period. Covers all intrauterine development from the moment of egg and sperm fusion (restoration of the diploid set).
- Postnatal period. Begins after the child is born.
The beginning of the prenatal period includes the cleavage stage of the zygote, which transforms into a blastula. The latter consists of the trophoblast (future placenta) and embryoblast (body of the embryo). This is followed by gastrulation with the formation of three germ layers (ecto-, ento-, and mesoderm) and subsequent histo- and organogenesis.
Classification of Prenatal Pathology
Depending on when the pathological factor affects the fetus, four forms of disorders are distinguished:
- Gametopathies. Occur before conception due to mutations in the parents' germ cells. They often lead to hereditary diseases, malformations, or early pregnancy loss.
- Blastopathies. Occur within the first 15 days after zygote formation. These include implantation defects (e.g., ectopic pregnancy), twin developmental anomalies, and the formation of empty gestational sacs. About half of embryos perish during this period.
- Embryopathies. Span from day 16 to day 75 (the phase of organogenesis). Most congenital malformations and teratomas form during this window.
- Fetopathies. Occur from day 76 until birth. They manifest as intrauterine growth restriction (IUGR), infections, metabolic disorders, hemolytic disease, and specific malformations (persistence of embryonic clefts or structures, or malposition of organs).
Main Causes of Pathologies
Fetal development can be influenced by a wide variety of factors:
- Genetic: gene and chromosomal abnormalities.
- Maternal: internal organ diseases (diabetes, heart defects, renal pathology), obstetric complications (preeclampsia, placental insufficiency).
- External teratogens: radiation, alcohol, tobacco, industrial poisons, and toxins.
- Infections: TORCH group and other microorganisms.
- Immune: blood incompatibility between mother and fetus (e.g., Rh incompatibility).
- Deficiencies: vitamin and micronutrient deficiencies in the pregnant woman.
Perinatal Terminology
Strict criteria are used to accurately assess newborn status and statistics:
- Live birth. An infant is considered live-born if, after extraction from the mother, there is at least one sign of life: spontaneous breathing, heartbeat, umbilical cord pulsation, or definite movement of voluntary muscles.
- Stillbirth. Fetal demise prior to complete expulsion or extraction. It can be antepartum (before the onset of labor) or intrapartum (during labor).
- Prematurity. Birth of an infant at a gestational age of less than 37 completed weeks. It is often accompanied by a birth weight of less than 2,500 g and a length of less than 45 cm, though low birth weight alone is not an absolute criterion for prematurity (it also occurs in term infants).
- Perinatal mortality rate. Reflects the number of stillbirths and neonatal deaths within the first 6 days of life per 1,000 total births.