Anatomical Changes in Multiple Gestations Compared to Singleton Pregnancies
Multiple gestations, particularly twin pregnancies, demonstrate significantly greater uterine distension, increased placental mass, and more pronounced maternal physiological adaptations compared to singleton pregnancies, with the specific anatomical differences heavily dependent on chorionicity.
Uterine and Placental Anatomical Differences
Uterine Size and Growth
- Twin pregnancies exhibit accelerated uterine growth and greater fundal height compared to singletons throughout gestation, with the uterus accommodating multiple fetuses and their associated amniotic sacs 1.
- The uterine cavity must expand to contain two or more fetuses, resulting in overdistension that contributes to the 5-fold increase in preterm labor seen in multiple gestations 1.
Placental Anatomy
- Dichorionic (DC) twins have two separate placentas (or a single fused placenta with two distinct portions), each with independent vascular supplies 1.
- Monochorionic (MC) twins share a single placenta with vascular communications between the two fetal circulations, creating unique anatomical risks not present in singletons 1.
- Placental surface area is significantly greater in twin pregnancies, contributing to the increased frequency of placenta previa, especially in dichorionic twins 1.
Membrane and Amniotic Cavity Configuration
Chorionicity-Specific Anatomy
- Dizygotic twins are always dichorionic-diamniotic, with two separate chorions, two amnions, and a thick dividing membrane between fetuses 1.
- Monozygotic twins have variable anatomy depending on timing of embryonic splitting:
- 1-3 days post-fertilization: dichorionic-diamniotic (two placentas, two amnions) 1
- 4-8 days: monochorionic-diamniotic (one placenta, two amnions, thin dividing membrane) 1
- 8-13 days: monochorionic-monoamniotic (one placenta, one shared amniotic cavity, no dividing membrane) 1
13 days: conjoined twins (shared anatomy) 1
Umbilical Cord Anatomy
- Velamentous cord insertion occurs in approximately 22% of monochorionic twin pregnancies compared to much lower rates in singletons, where cord vessels travel through membranes rather than directly into placental tissue 1.
- Marginal cord insertion is more common in twin pregnancies than singletons 1.
- Monochorionic-monoamniotic twins characteristically have entangled umbilical cords due to sharing the same amniotic cavity, which is pathognomonic for this type 1.
Cervical Anatomical Changes
- The cervix experiences greater mechanical stress from increased uterine distension in twin pregnancies, leading to higher rates of cervical shortening and incompetence 1.
- Cervical length assessment via transvaginal ultrasound is routinely performed to identify women at higher risk for preterm delivery, as the cervix shortens earlier and more rapidly than in singletons 1.
Maternal Physiological and Anatomical Adaptations
Cardiovascular and Hematologic Changes
- Twin pregnancies demonstrate more pronounced decreases in hemoglobin and hematocrit due to greater plasma volume expansion relative to red blood cell mass 2.
- Platelet counts are lower in twin pregnancies throughout gestation compared to singletons 2.
- Coagulation parameters differ significantly, with alterations in clotting factors reflecting the increased thrombotic risk 2.
Metabolic and Renal Changes
- Serum protein levels (including albumin) are lower in twin pregnancies due to greater hemodilution 2.
- Creatinine levels differ from singleton pregnancies, reflecting altered renal hemodynamics from increased cardiac output 2.
- Blood lipid profiles show distinct patterns in twin versus singleton pregnancies throughout gestation 2.
Fetal Anatomical Considerations
Growth Patterns
- Mean birth weight in twins (2.1 kg) is significantly lower than singletons (2.9 kg) at comparable gestational ages 3.
- Growth restriction is more common in multiple gestations, with discordant growth between twins being a unique anatomical concern 1.
Congenital Anomalies
- Structural anomalies occur twice as frequently in twin pregnancies compared to singletons, with monozygotic twins having 2-3 times greater risk than singletons 1, 4.
- Congenital cardiac anomalies are eight times more common in monochorionic-monoamniotic twins compared to monochorionic-diamniotic twins 1.
- Anomalies unique to multiple gestations include twin-twin transfusion syndrome (TTTS), twin reversed arterial perfusion (TRAP) sequence, and twin anemia-polycythemia syndrome (TAPS), all related to shared placental vascular anatomy 1, 5.
Clinical Implications of Anatomical Differences
Monitoring Requirements
- More frequent ultrasound examinations are mandatory due to anatomical complexities: first trimester dating scan, nuchal translucency at 11-14 weeks, anatomy scan at 18-22 weeks, and multiple third-trimester growth scans 1, 4.
- Monochorionic pregnancies require even more intensive surveillance (often every 2 weeks) due to shared placental anatomy and risk of TTTS 1.
Delivery Considerations
- Labor progression differs anatomically, with twin gestations having a significantly shorter first stage of labor (3.0 vs 4.0 hours) compared to singletons, likely due to lower fetal weight and different uterine contractility patterns 6.
- Cesarean delivery rates are higher (19.2% in twins vs 4.9% in singletons) due to anatomical factors including malpresentation and concerns about cord entanglement in monoamniotic twins 3.
Critical Pitfalls to Avoid
- Early first-trimester ultrasound (5.0-5.9 weeks) undercounts multiple gestations in 11-16% of cases, with monochorionic twins missed 86% of the time at this early stage 7.
- Failure to determine chorionicity early (ideally first trimester when accuracy approaches 100%) leads to inappropriate surveillance protocols, as chorionicity determines prognosis and management 1, 4.
- Overlooking vasa previa in the setting of velamentous cord insertion can result in catastrophic fetal hemorrhage at delivery 1.