Twinning & Epigenetics Flashcards
What is meant by “zygosity”?
- the zygote is the first structure that is formed post-fertilisation
- zygosity describes the number of zygotes that will be formed
Why has there been an upward trend in the prevalence of multiple births recently?
- increasing age of mothers at the time of birth
- increasing use of fertility treatments, including ARTs
What is meant by monozygotic, dizygotic and polyzygotic?
Monozygotic:
- this describes multiple pregnancies from a single zygote
- this produces identical twins (or more) as they originate from the same zygote
Dizygotic:
- this describes multiple pregnancies from 2 different zygotes
- this produces non-identical twins as 2 separate sperm have fertilised 2 separate oocytes to produce different zygotes
Polyzygotic:
- this describes multiple pregnancies from 2 or more zygotes
- this can produce a combination of both identical and non-identical twins
What % of twin pregnancies are dizygotic?
How does this occur?
- 90% of twins are dizygotic or “fraternal”
- there is simultaneous shedding of 2 oocytes and fertilisation by different spermatozoa
- the twins have a completely different genetic make-up so have no more resemblance than other brothers and sisters
- they can be the same or different sex
- the 2 ova do not have to be fertilised during the same act of sexual reproduction - it can occur on 2 different occasions
What factors can make twinning more likely to occur?
- increased levels of FSH can cause more than one dominant follicle to mature and ovulate
multiple oocytes are ovulated each menstrual cycle, making superfecundation more likely
superfecundation is the fertilisation of 2 or more ova from the same cycle by sperm from separate acts of sexual intercourse
- the likelihood of twins increases with age - incidence doubles in those > 35
- there is a correlation with increased parity (number of previous births)
- fertility procedures, such as ARTs, make twinning more likely as more than one fertilised oocyte is introduced into the reproductive tract
What is heteropaternal superfecundation?
How does it occur?
- this describes dizygotic twins that have different fathers
- it occurs when the mother has had sex with more than one man during ovulation where 2 ova were released
- more than one oocyte from the same cycle is fertilised (superfecundation) by 2 different sperm (dizygotic) to produce 2 zygotes
What is meant by superfoetation?
Why can it occur?
- this occurs when a second zygote implants into the uterus after another pregnancy has already begun
- these 2 different zygotes are technically not twins
- fertilisation occurring at a slightly different time point can lead to implantation occurring at a slightly different time point
What are the potential problems associated with superfoetation?
In what situation are these problems more likely?
- there is a marked delay in growth and maturation of the second foetus
- uterine contractions for one birth will influence the other birth during gestation, which can cause preterm birth of the second foetus
- if superfoetation occurs during the same menstrual cycle, both foetuses will be at a similar stage in development
- it causes more problems when fertilisation of the second oocyte occurs during a separate menstrual cycle
What is meant by monozygotic twinning?
- monozygotic twins occur when a single ovum is fertilised by a single sperm to produce one zygote
- there is then splitting of the zygote, which can occur at various stages in development
- It usually occurs at the early blastocyst stage
- this produces 2 individuals with “identical DNA”
What causes monozygotic twinning?
What influences the stage at which the zygote splits?
- the actual cause of monozygotic twinning is unknown
- the stage at which the zygote splits determines the extent to which the foetuses share membranes
- the later in development the splitting occurs, the more sharing of support structures there is
- it is thought that damage to the ICM could possibly lead to splitting of the early zygote
What has findings from IVF research found about possible causes of monozygotic twinning?
- damage to the zona pellucida prior to implantation may induce twinning
- blastomeres may become discordant (e.g. some may contain a mutation) and migrate away from “foreign” material
What are the 3 possible outcomes of triplets?
- fraternal / non-identical triplets occur when 3 separate ova are fertilised by 3 different sperm
- identical triplets can occur if a single zygote splits into 3 (rare)
- there can be one set of identical twins plus one that is non-identical
this occurs when there is fertilisation of 2 ova by 2 different sperm and then splitting of one of the zygotes
What are the possible combinations resulting in quads?
- 4 separate ova being fertilised by 4 different sperm
- 2 sets of identical twins - fertilisation of 2 ova which then both split
- 1 set of identical triplets + 1 non-identical - fertilisation of 2 ova with 1 splitting into 3
- 1 set of identical quads if a single zygote splits into 4
How is the extent of sharing of foetal membranes in monozygotic twins determined?
What is the risk of having only one amniotic cavity?
- the extent of sharing of foetal membranes depends on when in development the zygote splits
- if there is sharing of membranes, there needs to be adequate sharing of the blood supply to allow for perfusion of both twins
- if there is only one amniotic cavity, this reduces the space available and can lead to compression of the umbilical cord of one of the twins
How do the twins implant if there is splitting of the zygote at the 2-cell stage (day 0-3)?
- splitting early in development leads to the formation of 2 blastocysts
- the blastocysts implant separately into the uterine lining
- each blastocyst develops its own amniotic and chorionic cavity
- there is development of 2 placentas
- this is known as diamniotic and dichorionic twinning