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Hellin’s Rule (Incidence in India)

The mathematical frequency of multiple pregnancy is:

Twins                1 in 80

Triplets             1 in (80)2

Quadruplets      1 in (80)3 and so on



  1. Increasing age and increasing parity: The rate of natural twinning rises from 0 at puberty, a time of minimal ovarian activity, to a peak at 37 years of age, when maximal hormonal stimulation increases the rate of double ovulation. This is in accordance with the first consistently observed sign of reproductive aging, an isolated rise in serum FSH. The fall in incidence after 37 years of age probably reflects depletion of the Graafian follicles.
  2. Personal/family history of twinning.
  3. Treatment for infertility (ovulation induction agents/IVF).
  4. Negroes have the highest risk and Mongols and East countries have the least risk.



Twins can be of two varieties: dizygotic and monozygotic.

All dizygotic twins are: DC, DA (2/3)


Monozygotic twins: Incidence of monozygotic twins is constant throughout the world – 1 in 250.


Fetal sex: same.


Fertilization: one sperm, one egg

  1. Superfetation and superfecundation: In superfetation (Different cycle) , an interval as long as or longer than a menstrual cycle intervenes between fertilizations. Superfetation requires ovulation and fertilization during the course of an established pregnancy, which would theoretically be possible until the uterine cavity is obliterated by the fusion of the decidua capsularis to the decidua vera. Although known to occur in mares, superfetation is as yet unproven to occur in human.
  2. Superfecundation (same cycle) refers to the fertilization of two ova within the same menstrual cycle but not at the same coitus, nor necessarily by sperm from the same male.
  3. Sex ratios with multiple fetuses: In humans, as the number of fetuses per pregnancy increases, the percentage of male conceptuses decreases.
  4. Seventy percent of monochorionic-monoamnionic twins and 75% of conjoined twins are female.



Both vertex (most common)     60%


Vertex (1st) – Breech (IInd)      20%


Breech (1st) – Vertex (IInd)      10% (Interlocking is common)


Both Breech                              8 – 10%

Rarest lies is both the twins transverse.


Zygosity – Refers to the type of conceptions.


Twins can be:


Dizygotic twins (Two ova and Two sperm)


Monozygotic twins (One ova and one sperm & splitting)

Chorionicity – denotes the types of placentation.

  1. In dizygotic twins – each twin has its own placenta, chorion amnion i.e. dizygotic twins are always dichorionic diamniotic (i.e 2 chorion and 2 amnions)
  2. In Monozygous twins, the time at which the fertilized ovum divides – decides the Chorionicity.
  3. Chorionicity is of clinical significance as dichorionic twins, whether monozygous or dizygous, develop as two distinct individuals and are hence not at increased risk of complications. Whereas monochorionic twins are at increased risk because of the vascular anastomosis between the two circulations.
  4. Chorionicity can be detected prenatally by ultrasound (Best time – 6 to 9 weeks of gestation)

Monozygotic twins

< 3 days

4-8 days

>8 days

>14 days

Dichorionic diamniotic

Monochorionic diamniotic (most common)

Monochorionic monoamniotic

Conjoined twin

• Signs for chronicity on USG:




The "twin-peak" sign/lambda sign (placenta intervenes

"T" sign/Inverted T sign (right angle relation

between the membranes)

between the placenta and fetal membranes)

Intervening membrane is >2 mm thick

Intervening membrane is <2 mm


Ultrasound differentiation of Chorionicity








Fetal sex


Discordant / Concordant


< 2 mm

>2 mm

Number of layers in membrane

Two (2 amnion)

Four (2 amnion, 2 chorion)

Twin peak sign



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