Gestational Period
The entire duration of pregnancy, calculated from the first day of the mother’s last menstrual period (LMP).
Embryo
This stage refers to the developing organism during which all organ systems are recognizable, typically by Week 8. During the first 8 weeks, the structure grows rapidly from one cell to a 1-inch-long entity. The inner cell mass of the blastocyst forms the embryonic disc, which will eventually form the embryo.
Fetus
The developing human organism from weeks 9 through 38. Characterized by rapid growth of body structures established during the embryonic stage. Typically grows to 360 mm (14 in) and 3.2 kg (7+ lbs).
Neonatal Period
The first 28 days after birth, regardless of gestational age.
Capacitation
Process sperm must undergo before they can penetrate the oocyte. Motility is enhanced and membranes become fragile enough to release hydrolytic enzymes.
Zygote
The fertilized egg formed when sperm chromosomes combine with those of the secondary oocyte. A diploid cell that undergoes cleavage as it moves toward the uterus.
Morula
Cluster of 16+ cells formed about 72 hours after cleavage of the zygote.
Blastocyst
Formed around Day 4–5 when the 100-cell embryo reaches the uterus. Fluid-filled hollow sphere consisting of trophoblast cells and inner cell mass. Trophoblast secretes hCG.
hCG
Hormone secreted by trophoblast cells (later chorion). Promotes placental development and maintains the corpus luteum so progesterone and estrogen continue, preventing menstruation. Early pregnancy levels contribute to morning sickness.
Teratogens
Any factor that may cause severe congenital abnormalities or fetal death if it crosses the placenta. Includes alcohol (causes FAS) and nicotine (impairs oxygen delivery).
Sperm Capacitation
Sperm must be capacitated before penetrating the oocyte. Motility increases and membranes become fragile enough to release enzymes.
Acrosomal Reaction
Receptors in zona pellucida trigger enzyme release (hyaluronidase, acrosin, proteases) from many sperm to digest holes in the zona pellucida.
Cleavage
Rapid mitotic divisions of the zygote as it moves toward the uterus. Produces small cells with high surface-to-volume ratio. First cleavage at ~36 hours → two blastomeres
Implantation
Begins 6–7 days after ovulation. Trophoblast adheres, proliferates into two layers, and causes endometrium to become leaky. Blastocyst burrows in
Functions of the Placenta
Nutritive (nutrient transfer), respiratory (O₂ in/CO₂ out), excretory (waste removal), endocrine (hCG, estrogens, progesterone, hPL, relaxin), and barrier (prevents mixing of blood while allowing exchange).
Vascular Relationship at the Placenta
Chorionic villi extend into maternal blood. Umbilical vein carries oxygenated blood to fetus
Amnion
Transparent membrane from epiblast, filled with amniotic fluid. Cushions embryo and maintains temperature.
Yolk Sac
Hangs from ventral embryo. Forms part of digestive tube and earliest blood cells/vessels.
Allantois
Outpocketing of yolk sac
Chorion
Develops from trophoblast. Helps form placenta and encloses embryo and membranes.
Ectoderm
Forms nervous system and epidermis.
Mesoderm
Forms muscles, bones, circulatory system, kidneys, and gonads.
Endoderm
Forms epithelial linings of digestive, respiratory, and urogenital systems and glands.
Unique Features of Fetal Circulation
Umbilical arteries/vein, ductus venosus (bypasses liver), foramen ovale (bypasses lungs), ductus arteriosus (bypasses lungs).
