What Are the Odds of Having the Same Birthday as Your Sibling?

The easiest representation assumes that since there are 365 days in a year, each sibling has the same odds of 1/365 of being given birth on any of those days, and their deliveries are independent.

That means the odds that they have the same birthday is 1/365.

You can make changes to this model in several ways. For instance, you can include leap days.

For this model, you would possibly want to assume that the probability of being born on Feb 29th is one quarter (¼) of the probability of being born on any other day.

It complicates the analyses slightly and leads to a little lower probability as the result.

Ultimately, you could obtain some data on deliveries of children in a particular family.

You would discover that they repeatedly occur in the same season, and also in the same month.

That implies that the birthdays of siblings are not independent. You might discover that the odds of being born on the same day is 1/200 or more.

How often are siblings born on the same day?

The probability of a family giving birth to two siblings who have the same birthday is 1 in 133000

Let’s take for instance that the birth of a child in a home is likely to take place on any day, all through the year.

The child who came first can be delivered on any day, but the likelihood that the second is delivered on the same day is 1/365 (excluding leap years).

The possibility that the third is delivered on that day as well is 1/365, and supposing that there is no mutual factor connecting those birthdates then, the total probability of the three birthdays occurring simultaneously is 1/ (365 x 365 ) = 1 in 133000 or 7.5 in 1,000,000. And this is really small, making it rare.

There are several factors at play to define the precise odds of a mother giving birth to two children on the same date but in a different year. Some of these factors include:

The mother’s potency, the couple’s efforts to begin to give birth to their children on different dates, and the time it takes the couples to make babies.

However, a Statistics professor named Philip Stark from the same college disagrees with this figure.

He feels there are various factors at play to determine the exact odds of a woman bearing two children on the same date but in a different year.

Is it rare for siblings to have the same birthday?

Yes. It is rare for siblings to have the same birthday.

After giving birth, it takes about 6-12 weeks for fertility to return, this makes it unlikely for siblings to have the same birthday.

Although, there is an exception to that which is premature delivery If the second child at exactly nine months or less.

Child spacing is a personal choice made within the confinement of the couples.

Recently, couples have been made to see the side effect of birthing two children within the space of eighteen months.

There is a deficiency of nutrients in the mother’s body which is responsible for premature delivery and the weight of the newborn.

The advent of this recommendation has led to couples employing birth control measures while spacing their children.

What is it called when you have the same birthday as your siblings but a different year?

When you have the same birthday as your sibling, you are called an “Irish twin”.

They are not twins, because twins are carried in the womb concurrently and delivered at the same time. Irish twins are birthed within twelve months of having a baby.

The term “Irish twins” is used to describe the little age difference between siblings and was a renowned belittling slang against Irish Catholic people in the early nineteenth century.

This is because in the catholic faith, they believed employing birth control measures is sinful and so due to this belief, so many women got pregnant after giving birth.

In that dispensation, they disregarded having Irish twins because it portrayed that you have so many children you can’t cater for.

This ideology has changed as having Irish twins is on the increase as many women don’t want to have their kids later in life so they are in a rush to complete their family.

Others think that having their kids close in age will make them close emotionally.

Medical practitioners advise that women space their kids a little further apart because conceiving a baby within 9-12month after childbirth can lead to the second child having autism.

Can Siblings be 9 months apart?

Sure! Based on precise facts, Siblings can be nine months apart.

“Irish twins” is the term used to refer to siblings who are 9months apart.

In the old days, doctors assured women that there was no need for birth control measures since they had just given birth and are breastfeeding their babies, and as a result, are not liable to get pregnant during that period.

This guidance proved wrong as so many women eventually got pregnant and gave birth to either 9, 10, or 11 months after having a baby.

It was later studied and discovered that a woman can conceive another child after birthing one. This is dependent on how soon she has sexual intercourse.

Giving birth to two children in an interval of eighteen months has been linked with some hurdles during childbirth which include high chances of premature birth and the child’s weight below requirements.

What do you think are the odds of having the same birthday as your mother?

The probability of having the same birthday as your mother is 1 out of 365.

Most people are controversial about this because they think having the same birthday as your mother is easy-peasy.

And here’s why they think so, if your mum’s birthday is on 5th June, and she conceives you on 9th March which is a nine months interval then you should be given birth to on 5th June.

This is true with all things being equal but this is reality and there are a lot of factors that may hinder this from coming through.

Factors such as medical complications include forced labor or prolonged labor.

Therefore, the odds of an individual being born on any day of the year regards the day (whether your mum’s birthday or not) is 1 in 365.

What is the most common birthday?

The most common birthday on the planet is September 9th. On this day from 1994 to 2004, there was an average of 12,301 birth in the US alone.

A person whose birthday falls on September 9th was conceived on 17th December the previous year.

Asides from September 9th being the most common birthday, among the 10 most popular birthdays in the world, nine of them are in September.

September 19th is the second most popular birthday.  From 1994 to 2014, an average number of 12,229 deliveries were recorded on September 19th.

Hypothetically, those who were born on this day were conceived on conceived 27th of December the previous year.

September 12th is the third most popular birthday. An average of 12,225 births have been recorded on this day.

20th December to 12th September is a period of months which means 20th December is the day they were conceived.

September 17th is the fourth most common birthday with an average of 12,148 persons were given birth to.

Interestingly, these individuals were conceived on 25th December ( Christmas Day).

We may not be able to exhaust all the 9 most popular birthdays in the month of September, let me run you through the rest.

September 10th – 5th most popular birthday

September 20th – 7th most popular birthday

September 15th – 8th most popular birthday

September 16th – 9th most popular birthday

September 18th – 10th most popular birthday.


Birthdays are wonderful moments which almost everyone celebrates, even if without a grand celebration, it is acknowledged as a milestone.

It is even more thrilling to share the same birthday with a sibling or someone you are not related to as both of you are considered a type of twin.

Sometimes parents intentionally decide on birthing two or more kids immediately after another.

Some feel it will save them from making more expenses during their birthdays while others feel it will make the kids more emotionally attached to themselves.

Every couple’s motive to birth Irish twins is understandable but there are medical implications that make the mother or the child liable for complications.

This is why adequate spacing between siblings is advised. As a parent, you will not want to give birth to a child who has autism because of your quest to have matching birthdays with their siblings.

In as much as Irish twins are adorable, you would want to look on the less complicated side and give your children the recommended spacing of at least 12months.


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