In 99.9% of cases boy/girl twins are non-identical. However, in some extremely rare cases resulting from a genetic mutation, identical twins from an egg and sperm which began as male (XY) can develop into a male / female pair.
Monoamniotic-monochorionic Twins
This is the rarest type of twin, and it means a riskier pregnancy as the babies can get tangled in their own umbilical cords. If you have monoamniotic-monochorionic twins, your healthcare provider will monitor your pregnancy closely.
The majority of triplets are what is known as trizygotic. This is because they are formed when three separate, individual eggs are fertilised by three separate sperm. These babies share the same characteristics as any other siblings with the same biological parents. They can be the same or opposite genders.
Trizygotic or fraternal triplets are formed when three separate eggs are fertilised by three separate sperm. The babies can be the same or opposite genders. This means that it's possible to have anyone of the following combinations of babies – three girls/three boys or, one girl and two boys or two girls and one boy.
It is possible to have triplets where two of the babies are identical twins (and may share one placenta, and even one sac) and the third baby is non-identical (with completely separate placenta and sac).
The first wife (died ante 1770), of Feodor Vassilyev (b. 1707–c. 1782), a peasant from Shuya, Russia, gave birth to 16 sets of twins.
Superfetation involves two embryos that form during two separate menstrual cycles. With superfecundation, your body releases two or more eggs during the same menstrual cycle. Each egg is fertilized by sperm, which can be from the same or a different partner or donor. A note from Cleveland Clinic.
The DNA of monozygotic twins tends not to be 100% identical, and epigenetic and environmental differences further widen the gap between twin pairs. It's not nature or nurture; it's a complex interaction between our genes, our environment, and our epigenetic markers that shape who we are and what illnesses befall us.
It is rare that one twin is born with Down syndrome and the other is not; the Franzes' pediatrician stated that she was only aware of three other cases. The Franz family knows that their girls are unique, including their big brothers.
Even identical twins – who have the same DNA sequence and tend to share a very similar appearance – have slightly different fingerprints. That's because fingerprints are influenced by both genetic and environmental factors during development in the womb.
"Before it was like 'how are we going to have two babies,' and now it's like 'two babies, that's nothing!
Once the babies are large enough to stay in one position in the womb, the twin lowest in the uterus is known as Baby A and the one furthest from there is Baby B, according to the Stanford Medicine News Center. In the majority of vaginal births, Baby A is born first.
The longest interval between the birth of twins is 90 days, in the case of Molly and Benjamin West, dizygotic (fraternal) twins born in Baltimore, Maryland, USA to parents Lesa and David West (all USA) on 1 January and 30 March 1996.
Race. African-American people assigned female at birth are more likely to have twins than any other race. Asian Americans and Native Americans have the lowest rates for twins. White people assigned female at birth have the highest rate of higher-order multiple births (triplets or more).
What is it? 'Mirror image' is a type of identical twinning. It can happen in any type of identical twins. When the split occurs late - more than a week after conception - the twins can develop reverse asymmetric features. This term is not really a type of twin, just a way to describe their physical features.
The technical name for this is dichorionic. Fraternal twins can be the same or opposite sex and their genes are as different as any other brother and sister. Often, same-sex fraternal twins look different. For example, they might have different hair or eye colour.
Identical twins are almost always the same sex, although there are some rare exceptions to this rule. Because identical twins occur when a single fertilized egg splits and forms two embryos, each embryo has the same chromosomes (usually, XX for girls or XY for boys).
Nobody knows what causes identical (monozygotic) twins. Everyone has the same chance of having identical twins: about 1 in 250. Identical twins do not run in families.
The notion that twins always skip a generation is also a myth. The illusion may have arisen because men who inherit the gene from their mothers are unaffected by it ( they do not ovulate), but can still pass it on to their daughters, who, in turn, will have an increased likelihood of conceiving twins.
The Dionne quintuplets (born 28 May 1934, near Corbeil, Ontario, Canada) were the first quintuplets known to survive infancy. The five girls (Yvonne, Annette, Cecile, Emilie and Marie) were also the only set of identical quintuplets known to live into adulthood.
The survival rate for monoamniotic twins is somewhere between 50% to 60%. Monoamniotic twins, as with diamniotic monochorionic twins, have a risk of twin-to-twin transfusion syndrome.