Twinning

Question 1

How do dizygotic twins arise?

Answer 1

2 eggs that are separately fertiilized by 2 different sperm. Always two zygotes.

Question 2

How do monozygotic twins arise?

Answer 2

1 fertilized egg (1 egg + 1 sperm) that splits into 2 zygotes in early pregnancy. Degree of separation depends on when fertilized egg splits.

Question 3

What % is mono vs. dizygotic?

Answer 3

80% dizygotic, 20% mono.

Question 4

If zygote splits between the 0-4 days, at what stages of embryonic development is the zygote? What % of monozygotic twins split here?

Answer 4

Between 2 cell and morula. 25%.

Question 5

If zygote splits at 0-4 days, what degree of separation will the twins have?

Answer 5

Dichorionic (two placentas) diamniotic. Placenta may be fused or separate

Question 6

If zygote splits between 4-8 days, at what stages of embryonic development is the zygote? What % of monozygotic twins split here?

Answer 6

Between morula and blastocyst. 75%.

Question 7

If zygote splits at 4-8 days, what degree of separation will the twins have?

Answer 7

Monochorionic, diamniotic.

Question 8

If zygote splits between 8-12 days, at what stages of embryonic development is the zygote? What % of monozygotic twins split here?

Answer 8

Between blastocyst and formed embryonic disc. <1%

Question 9

If zygote splits at 8-12 days, what degree of separation will the twins have?

Answer 9

Monochorionic, monoamniotic.

Question 10

If zygote splits >13 days, at what stages of embryonic development is the zygote? What % of monozygotic twins split here?

Answer 10

After embryonic disc has formed.

Monochorionic, monoamniotic conjoined twins.