One of the best explanations for this is something called delayed interval birth. Sometimes, during twin or multiple pregnancies, a complication occurs with a baby that doesn't with the others.
In other words, they were not born during the same gestational period. Calling them Irish twins is just an informal way of classifying siblings that are born close together. Some actual twins have different birthdays due to one being born on one day and the other staying in the womb for days or weeks longer.
One in 2 million: Twins born on different day, month and year - The Jerusalem Post.
Fraternal or 'dizygotic' twins
These babies will be no more alike than siblings born at separate times.
For the most part, twins and multiples share the same birthday. However, depending on the time of day the babies are born and how long the timespan is between each baby's birth, twins can be born on different days.
DEFINITION: Two people born at the same date and time. They are often seen to have very similar personalities and may even have physical appearances that make them look like twins.
Mirror twins, or mirror image twins, is a term used to describe a characteristic of some twins whose features appear asymmetrically—that is, on opposite sides. When these twins are facing each other, it is as if they are looking in a mirror.
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.
February 3rd is the only day where no one in history has ever been born. Despite much scientific study, there is no explanation for this phenomena. Historically it has been referred to as “the empty day” or “nobody's birthday”.
Twins with different birthdays are rare, and the chance of twins being born in different years is 1 in 2 million, according to the news release by Natividad. Though the incident is rare, it's not the first time twins have been born in separate years.
The term “Irish twins” refers to siblings who are born within 12 months of one another. This is especially true for children born in the same calendar year, or children who would be in the same grade of school. However, children born within 18 months of one another are still often referred to as Irish Twins.
The term, which is far less common today than it was in years past, is sometimes used to describe two children born to the same mom within a span of 12 months or less, though some people consider siblings born within 18 months of each other to be Irish twins too.
Twin reversed arterial perfusion (TRAP) occurs in identical twin pregnancies when one twin has an absent or non-functioning heart and receives all of its blood from the normally functioning and developing "pump twin." The twins are joined by a large blood vessel between their umbilical cords.
Such twins, known scientifically as 'MoMo', an abbreviation for monoamniotic-monochorionic, are some of the rarest types of twins, making up less than one percent of all births in the United States, noted the statement. It also stressed that such MoMo twin pregnancies have a high risk of fetal complications.
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.
Some even believe that strangers could have nearly identical DNA, like twins from birth. In depth research has not been completed on the topic, but most experts agree that it is possible that everyone has a doppelganger, maybe even more than one.
Heteropaternal superfecundation is an extremely rare phenomenon that occurs when a second ova released during the same menstrual cycle is additionally fertilized by the sperm cells of a different man in separate sexual intercourse.
It's also possible to have twins that were conceived at different times in a process called superfetation. In exceedingly rare cases, a woman might release two eggs during ovulation, but not at the same time. In such cases, twins may actually be born on different days.
Superfecundation is the fertilization of two or more ova from the same cycle by sperm from separate acts of sexual intercourse, which can lead to twin babies from two separate biological fathers.