Fertilisation and Implantation
Fertilisation: fusion of haploid egg and sperm, forming diploid zygote
If a sperm combines with an empty egg > Molar pregnancy
If a single sperm fertilises an egg > Singleton pregnancy (potential for monozygotic (MZ) aka identical twins)
(Hypothesised 3rd twin type: egg splits then fertilised by different sperm: identical fraternal twins)
If a multiple sperm fertilise seperate eggs > fraternal aka dizygotic (DZ) twins (all are DCDA)
If multiple sperm single egg = polyspermy (i.e. 3+ x each chromosome) unviable
Assisted fertility: IVF, Drugs
Somatic cell nuclear transfer (Cloning): putting donor nucleus in enucleated oocyte
Cleavage:
The timing of cleavage (to twins) defines the type of monozygotic twin
Dichorionic (as with DZ, placentas can fuse to look singular, but no TTT)
If a sperm combines with an empty egg > Molar pregnancy
If a single sperm fertilises an egg > Singleton pregnancy (potential for monozygotic (MZ) aka identical twins)
(Hypothesised 3rd twin type: egg splits then fertilised by different sperm: identical fraternal twins)
If a multiple sperm fertilise seperate eggs > fraternal aka dizygotic (DZ) twins (all are DCDA)
If multiple sperm single egg = polyspermy (i.e. 3+ x each chromosome) unviable
Assisted fertility: IVF, Drugs
Somatic cell nuclear transfer (Cloning): putting donor nucleus in enucleated oocyte
Cleavage:
The timing of cleavage (to twins) defines the type of monozygotic twin
Dichorionic (as with DZ, placentas can fuse to look singular, but no TTT)
- (MZ)DCDA occurs with division prior to the morula stage (<3d)
- (MZ)MCDA (d4-8)
- (MZ)MCMA (d8-12)
- Conjoined twins (MZMCMA) (>13d)