Multiple Pregnancy

Question 1

What are the two types of twins you can get?

Answer 1

Dizygotic and monozygotic

Question 2

Are multiple pregnancies increasing?

Answer 2

Yes

Question 3

What occurs to get dizygotic twins?

Answer 3

Two sperm fertilises two seperate eggs and they are no more related than non-twin siblings

Question 4

What makes a monozygotic set of twins?

Answer 4

One egg and one sperm. The egg splits after fertilisation - they share all DNA and appear identical

Question 5

What are the types of triplets you can get?

Answer 5

One zygote - all identical
Two zygotes - one pair of identical twins and one non identical
Three zygotes - three sperm

Question 6

What is chorionicity?

Answer 6

Twins can have different types of chorions depending on when they split.

Question 7

What type of chorions and amnions does a dichorionic diamniotic twins have?

Answer 7

Two chorions, two amnions (one each) they also have different placenta

Question 8

What amnions and chorions do monochorionic diamniotic twins have?

Answer 8

One chorion, two amnions and share one placenta

Question 9

What amnions and chorions do monochorionic monoamniotic twins have?

Answer 9

One chorion one amnion and one placenta to share

Question 10

What type of chorionicity are monozygotic twins?

Answer 10

All - monochorionic monoamniotic/ monochorionic diamnioitic/ dichorionic or diamniotic

Question 11

What type of chorionicity do dizygotic twins have?

Answer 11

Only dichorionic diamniotic

Question 12

Can the placenta fuse in dizygotic twins?

Answer 12

Yes

Question 13

How can monozygotic twins have different chorionicity?

Answer 13

Depends on the time they split.

If the eggs split day 1 - 3 no structures have been made and so they will make their own and be DC/DA

If the zygote splits day 4 - 8 they will have seperate chorions but share an amnion (MC/DA) as the amnions develop before the chorion.

Question 14

What happens if twinning occurs after day 13?

Answer 14

Conjoined twins

Question 15

What are conjoined twins?

Answer 15

Twins which are joined together somewhere on their body due to not correctly splitting

Question 16

What is twin to twin transfusion syndrome?

Answer 16

Happens is MC/DA pregnancies and involves the placenta vascularising so that one twin gets more blood than the other

Question 17

What are mirror image twins?

Answer 17

Happen in MZ twins when they split as the cells become opposite on another - this means they both look alike but minor features are on opposite sides e.g handedness, wrinkles etc.

Question 18

Has the multiple birth rate increased?

Answer 18

Yes - its more common in Africa

Question 19

Does IVF cause an increase in multiples?

Answer 19

Yes

Question 20

Why does ART increase the amount of multiple births?

Answer 20

Because more embryos are put back and also the drugs used can increase ovulation.

Question 21

Is there a campaign to try and stop multiple pregnancies through IVF?

Answer 21

Yes as multiple births are dangerous

Question 22

do all countries only put 1 - 3 embryos back?

Answer 22

no america will put up to 8 back

Question 23

Why does IVF increase the chances of having identical twins and not non-identical twins?

Answer 23

The zona pellucida might alter (hardening) depending on the drugs being used and therefore might be more likely to split.

Question 24

Does increased age makes you more likely to get multiple births?

Answer 24

Yes - over 35 = twice as likely to have twins

Question 25

Why might older mothers have twins?

Answer 25

s someone gets older there is less follicles and less inhibin which increases the amount of FSH which could influence how many eggs ovulate

Question 26

Could increasing the amount of ovulation be an evolution strategy?

Answer 26

Yes as in older women there is less chance of young surviving and so if you have 2 you might get 1 live birth. But obviously if they are both healthy you get twins

Question 27

What makes you more likely to have dizygotic twins?

Answer 27

Common in some ethnic groups
DZ runs in mothers side of the family

Question 28

How could genetics influence twins?

Answer 28

Ovulation is controlled by FSH, GDF9 and BMP15. Mutations in either BMP15 and GDF9 has been associated with increased DZ twins.

Sequence variation of FSH and SMAD3 increases chance of DZ in women

Question 29

Are there genetic causes of MZ twins?

Answer 29

Not really as they don’t run in families however familial MZ twins can transmit from mother or father (suggests an inherited defect) and mutation in zona pellucida leads to failure/delay of cell to cell recognition or adhesion

Question 30

What are semi identical twins?

Answer 30

A first trimester twin scan showed a monochorionic diamnotic twins (should be identical).
The second scan showed twin A was male and twin B was female.
They did a genetic analysis and found that they shared 75% DNA - an identical maternal allele but different paternal alleles.

Question 31

How do semi identical twins develop (Sesquizygous twins)

Answer 31

Two sperm fertilise an egg creating 3 cells (this should only be two) The two cells divide and a twinning event occurs - one takes an XY the other XX (the twins are chimeras)