L15: Multiple Pregnancy Flashcards
What are the different types of twins?
- Dizygotic (DZ)
- monozygotic (MZ)
What are the differences between monozygotic and dizygotic twins?
monozygotic:
- 1/3 of twins
- one egg and one sperm
- zygotes are genetically nearly identical
- share all DNA
- twins appear identical
- always same sex
dizygotic:
- 2/3 of twins
- two eggs and two sperm
- zygotes have different genetic constitutions
- share 50% of DNA
- twins have no more resemblence than any brothers or sisters
- can be of different sex
What is the frequency of monozygotic and dizygotic twins?
- monozygotic - 1/3 twins
- dizygotic - 2/3 twins
How much are genetically monozygotic and dizygotic twins similar?
monozygotic - genetically nearly identical, share all DNA
dizygotic - different genetic constitutions, share 50% of DNA
What are the possible sexes of monozygotic and dizygotic twins?
- monozygotic always same sex
- dizygotic can be same or different sex
How can triplets be derived?
- one zygote and all be identical
- two zygotes, consisting of identical twins and a singleton
- three zygotes and be of the same sex or of different sexes
How do twins develop in the uterus? (chorionicity)
- monochorionic monoamniotic (share chorion and amnion; MC/MA; share placenta)
- Monochorionic diamniotic (share chorion, but two amnions; MC/DA, share placenta)
- Dichorionic diamniotic (two chorions, two amnions; DC/DA; everything separate)
check L15, slide 6
What is the most frequent chorionicity of monozygotic and dizygotic twins?
- for monozygotic most frequent is monochorionic diamniotic
- for dizygotic it’s always dichorionic diamniotic
How is chorionicity determined?
The degree of splitting is determined by the timing of the split:
- at days 1-3 - DC/DA
- at days 4-8 - MC/DA
- at days 8-13 - MC/MA
When does chorion and amnion post-fertilisation begin to form?
- chorion forms 4-5 days post-fertilisation
- amnion begins to form on day 8
How are two complete and separate chorions/amnions determined?
Twins who have two complete and separate chorions indicates that separation occured before the formation of the trophoblast tissue on day 5
How are separate amnions but same chorion determined?
In pregnancies where twins share a chorion, suggests that split occurred within the ICM after the trophoblast formed. If they have separate amnions: suggests the split occured before the formation of the amnion (day 9)
How are shared amnion and chorion determined?
If separation of ICM comes after formation of chorion and after the formation of amnion then resulting embryos would have one chorion and one amnion
What are conjoined twins?
The two fetuses will remain physically connected (most often at the chest, abdomen or pelvis), conjoined twins may also share one or more internal organs
How does conjoined twinning occur?
if separation of an embryo occurs between day 13 & 15 & the inner cell mas does not completely separate, or if portions of the inner cell mass secondarily re-join, conjoined twins may result