Anatomy Unit: Umbilical Cord & Twins

Umbilical Cord Development and Structure

• Connects fetus to placenta, approximately 50-60 cm in length and 1 cm in diameter.
• Twisted shape due to the left umbilical vein being shorter than the arteries; features false knots.
• Amniotic membrane covers both surfaces, with central attachment to the placenta.
• Composed of two umbilical arteries (non-oxygenated blood) and one left umbilical vein (oxygenated blood).
• Additional structures: Wharton's jelly (gelatinous substance), remnant of vitello intestinal duct (disappears by the 3rd month), and definitive yolk sac (disappears by the 5th month).

Umbilical Cord Anomalies

• Variants in number: possible to have double or triple cords.
• Length anomalies: very short or very long cords can lead to complications.
• Types of knots: false knots have no clinical significance, while true knots can tighten and potentially cause fetal death.
• Attachment issues: marginal (away from center), velamentous (attached to amnion).
• Structural anomalies may include absent artery due to agenesis or degeneration, associated with cardiovascular abnormalities (15-20% incidence).

Definition and Incidence of Twins

• Twins defined as delivery of more than one fetus, occurring in 1 in every 85 pregnancies.
• Incidence for triplets and quadriplets is 1 in every 90 pregnancies.
• Causes include hereditary factors and medical interventions like gonadotrophins or in vitro fertilization.

Types of Twins

• Identical twins (monozygotic): arise from one fertilized oocyte, occur in 30% of twins, genetically identical.
• Non-identical twins (dizygotic): result from fertilization of two oocytes by two sperms, making up 70% of twins, can be of the same or different sex.

Monozygotic Twins Detailed

• Separation in the 2-cell stage results in two placentas, chorions, and amnions.
• Separation in the inner cell mass produces one placenta, one chorion, but two amnions.
• Bi-laminar disc division yields equal or unequal embryos, leading to one placenta and one chorion with one or two amnions.

Conjoined Monozygotic Twins

• Result from incomplete division of the embryonic disc.
• Types include thoracopagus (most common, united at the thoracic region), craniopagus (united at the head), and pygopagus or ventral body wall joins.

Case Scenario Quiz

• Queries about placental components of dizygotic twins:
  • Correct answer is (D) Two placentas, two amniotic sacs, and two chorions.