For the most part, twins and multiples share the same birthday. Twins are defined as two offspring born together, but that doesn't necessarily mean they are born on the same date. Multiples are generally born only a few minutes apart. If delivered by cesarian section, the interval between births is usually only a minute, maybe two.

The interval is generally longer for vaginal deliveries, ranging from a few minutes to more than an hour. In modern times, if the time period extends much beyond that, doctors elect to perform a C-section for the second baby, fearing distress or the risk of infection. However, some studies make a case against this practice.﻿﻿

There are exceptions to the rule that twins have the same birthday. Some are due to a delivery near midnight, while others may extend to longer intervals due to medical intervention delaying the interval of births.

Delayed Interval Births

In some cases, a delayed interval birth takes place when one multiple is delivered prematurely, but doctors are able to halt labor and keep the remaining multiple(s) in their mother's womb to grow and develop. Some of these multiples—twins, triplets or more—were born days, weeks or even months apart. These are rare events.﻿﻿

Born on Either Side of Midnight

The difference between today and tomorrow is literally a matter of seconds. Plenty of babies are delivered in the middle of the night. Babies that are born close to midnight may arrive on different days. For example, Twin A may be born at 11:59 pm on one day while Twin B doesn't make an appearance until 12:01 am, which would be the next day. This can create some interesting dilemmas.

For example, two British twins were born either side of midnight on August 31 and September 1. Unfortunately, September 1 is the cut-off date for starting school in their area, so only the first-born twin would turn 5 in time for the start of kindergarten. The father, a twin himself, said he would do everything possible to ensure they both were in the same grade at school.

The difference in the month would also matter when they were due for immunizations, even though there were born only minutes apart. While common sense may rule, sometimes there are strict date cutoffs encoded in law that would require petitioning for an exception.

Born in Different Years, Decades, and Millennia

There are cases of twins born in different years. In Texas, twins were born only one minute apart, but in different years, delivered by C-section as the last baby of 2013 and the first baby of 2014. A similar situation occurred as 2015 changed to 2016 in San Diego with fraternal twins.

Twins can be born in different decades, as happened as 2009 changed to 2010 and a pair of Florida twins came into the world on each side of the dividing line.

To top it all, there are twins born in different millennia. Twins were born in Massachusetts only eight minutes apart, but they were born in proximity to the turn of the millennium in 2000. One boy arrived on New Year's Eve, 1999, just a few minutes shy of midnight. And in the first few seconds of the year 2000, his twin brother took his first breath.

In these cases, there can be ongoing implications for eligibility for programs and services as well as tax deductions. The parents would have to advocate for including both twins the year the first was eligible or delaying until both were eligible, if possible.