Multiple Births: Causes, Types & Contributing Factors

Why Multiple Pregnancies Occur

A multiple pregnancy (twins, triplets, more) results from either multiple ovulations or splitting of a single zygote.

1. Dizygotic (Fraternal) Multiples – Separate Eggs, Separate Sperm

• Multiple ovulation: Ovaries release >1 oocyte in a cycle.
• Fertility treatments: Ovulation induction (clomiphene, letrozole, gonadotropins) → ↑ multiple ovulation; IVF with multiple embryo transfer.
• Genetics: Maternal family history of DZ twins (hyperovulation genes, e.g., FSHB, SMAD3). Paternal history irrelevant.
• Maternal age ≥35: ↑ FSH → ↑ multiple follicular recruitment.
• Race/ethnicity: Highest DZ twinning in Black populations (Nigeria: ~40/1000); lowest in Asian.
• High parity, tall stature, high BMI – modest associations.

2. Monozygotic (Identical) Multiples – Single Zygote Splits

• Spontaneous, random event (~3–4/1000 births); cause unknown.
• Timing of split determines chorionicity/amnionicity (di-di, mono-di, mono-mono).
• Not hereditary (rare familial clusters reported but no confirmed gene).
• Slightly increased with IVF/ICSI (assisted hatching, blastocyst culture).

3. Higher-Order Multiples (Triplets+)

• Almost always fertility-treatment related (multiple embryo transfer, superovulation).
• Spontaneous triplets: ~1/8000 (usually DZ+MZ combination).

Risks & Management

• Preterm birth (60% twins <37 wks; 90% triplets <34 wks)
• Preeclampsia, GDM, anemia, postpartum hemorrhage
• Twin-twin transfusion syndrome (monochorionic)
• Specialized MFM care, cervical length surveillance, corticosteroids, timed delivery.

Understanding zygosity and chorionicity guides prenatal management.