Lec 7 - Twins

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What are twins?

i) Definition

Two offspring produced by same pregnancy

ii) Types

Identical (monozygotic)

Fraternal (dizygotic)

Conjoined

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How common are twins?

~ 1/100 births

Most common among African Americans (1/70)

Least common among Chinese (1/300)

Triplets = ~ 1/750'000 births

Quadruplets = ~ 1/650'000 births

Incidence of twins/triplets/quadruplets higher in IVF

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Identical monozygotic twins

i) What are identical monozygotic twins?

One egg fertilised by one sperm

Resulting zygote splits into 2 separate individuals

Offspring have identical genetic information!

ii) Dichorionic diamniotic

Split occurs before cells differentiate at 4 days (Still Morulas)

Occurs whilst travelling down oviduct

Will implant as 2 seperate blastocysts

Dichorionic:

2 separate chorions (seperate placentas!)

Diamniotic:

2 separate amniotic sacs

Decreased risk of entanglement & twin-twin syndrome

Occurs in 1 out of every 4 sets of twins

iii) Monochorionic diamniotic

Split occurs between days 4-8 (Blastocyst stage

Monochorionic:

1 shared chorion (1 placenta between 2!)

Increased risk of one foetus 'hogging' maternal nutrients

Diamniotic:

2 separate amniotic sacs

iv) Monochorionic monoamniotic

Split occurs between 8th & 13th day

Implant into endometrium as one blastocyst

THEN split into 2!

Monochorionic:

​​​​​​​1 shared chorion (1 placenta between 2)

Monoamniotic:

​​​​​​​1 shared aminotic sac

Increased risk of entanglement/strangulation due to umbilical cords

​​​​​​​Risk decreases as twins grow larger - less room & movement

Twin-twin syndrome

Mo-mo twins share a placenta

May lead to abnormal blood vessel connection development

Imbalance of blood flow

One twin may end up receiving more blood than the other

Twin that loses blood = dehydrated & produces less urine

Often smaller

Twin that gains blood = develops high BP & produces more urine

​​​​​​​Extra blood may lead to heart failure

If untreated can cause termination of one or both pregnancies

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Conjoined twins

Remember day 13 = differentiation of embryonic disc (**See CNS embryology lecture pg.4 iii) Inner cell mass differentiation**)

If split occurs after day 13 - Mo-mo twins form

HOWEVER, they will also share body parts

Only monozygotic twins can be conjoined

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Fraternal twins

More than one egg is released during ovulation

Multiple sperm fertilise multiple eggs

Each offspring = unique in terms of genetic makeup

Same as siblings born of different pregnancies

Can be same or different sex

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