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Why is September the most common birth month?

Have you ever found yourself marveling at how many friends and family members have birthdays in September? Did you know September is the most popular birth month across America? That’s right: More babies are born every day in September than in any other month of the year.

So why is September the most common birth month? Turns out it has a lot to do with 9 months before that, in December — the month where couples are most likely to conceive.

Key takeaways:

  • September is the most popular birth month, making the highest rate of conception in December.
  • This birth rate in September is likely affected by the increased level of sexual activity over holiday periods nine months before September, but is also due to seasonal changes in human fertility that reflect changes in temperature and weather.
  • If you’d like to have a baby next year, there are steps you can take now to prepare your body, optimize your fertility, and increase your chances of getting pregnant (or getting your partner pregnant).

Ranking the most popular birth months

In the US, more babies are born daily during September than during any other time of year.

According to CDC statistics from 2010, these are the most common birth months:

  • An average of ~11,700 babies were born each day in September
  • An average of ~11,300 babies were born each day in August
  • An average of ~11,100 babies were born each day in July

These are the top 3 months of the year for births. The least popular birth months? Winter months.

  • An average of ~10,800 babies were born each day in February
  • An average of ~10,400 babies were born each day in January

All but one of the top 10 most common birthdays falls in the month of September.

What month is a person most likely to get pregnant?

It would appear that, in most regions of the world, December is the most fertile month of the year. As you likely know, pregnancy lasts for around nine months, or 40 weeks. So most of the full-term babies born in September were conceived in December.

Is it a coincidence that winter holidays like Christmas fall 9 months before many babies are born? It would appear not — it’s thought the high number of births in September is associated with an increase in alcohol intake, time off, and partying at Christmas, which have all been linked with an increased number of sex sessions.

However, we’ve also been known to party, take time off, drink, and have sex in the summer months. So what’s the difference? Why are conception rates so much higher in December?

A study published in the Journal of Obstetrics & Gynecology found evidence to suggests couples actually have a higher chance of conceiving in winter. An analysis of over 1 million births indicated that births are much more likely to occur in the summer months as the winter season holds some “biological perks” that support pregnancy. Let’s explore these seasonal effects on fertility and find out why December is the month you’re most likely to get pregnant.

What affects fertility from month to month?

Researchers are investigating the seasonal effects on conception and birth rates, to understand the impacts of environmental factors on fertility and to learn more about how couples can optimize their chances of conceiving.

One study has highlighted that humans have seasonal variations in hormones involved in thermoregulation, metabolism, stress adaptation, growth, and reproduction. We also experience changes in the immune system that could play a role in birth rate seasonality.

Some of the primary factors thought to be influencing seasonal variations in conception are:

  • Temperature of the environment
  • Frequency of sex
  • Social planning

Weather, temperature, and fertility

We know that sperm and semen quality is influenced by scrotal temperature, which is why men are advised to adjust their underwear choices during the hot months especially, to allow for cooler scrotal temperatures. 

Recent evidence suggests there is a deterioration in sperm quality during the hot summer months in sub-equatorial areas. This may lead to a reduction in the birthrate for winter and spring. Evidence suggests that humans have seasonal fertility peaks around the winter solstice, whether or not this falls around a festive period, indicating that temperature and seasonal variations have more of an influence on conception rates than the increased frequency of sex.

Fluctuations in the frequency of sex

Sexual activity does increase during the festive periods around Christmas and the New Year celebrations and will ultimately impact the number of successful fertilizations. By analyzing app user data, one study found that sexual activity is consistently elevated over weekends and during holiday periods, leisure times, and romantic celebrations such as Valentine’s Day, corroborating earlier findings about how culture and collective moods influence human sexual cycles. Sometimes it is just a case of more sex = more pregnancies.

Social planning for pregnancy

Studies have shown that babies conceived in December have a better chance of being born healthy compared to those conceived during the summer months. As a result, some families may want to plan for a summer birth to increase their chances of a healthy baby. It is thought this may be related to vitamin D levels in the mother being higher in summer-born babies, due to increased sunshine exposure during the later stages of pregnancy.

Some parents feel it is beneficial, educationally, to have a child that is among the oldest in their class, rather than an early summer-born baby, who might be one of the youngest in their year group. In the UK, children begin school in September after they turn 4 years old. To this end, some parents plan to try for a baby around Christmas, thus ensuring their baby will be born after the school year cut-off date.

It’s also possible that some families actively try to spread birthdays out if they have several children, so as not to have all their birthdays at once. When family planning, people sometimes actively choose to avoid having a baby around times like Christmas or other significant social events. 

If I want to have a baby next year, what can I do now?

If you are planning to have a baby next year, there are many things you can start doing now that will not only improve your chances of becoming pregnant but could positively impact you and your baby’s health.

1. Get yourself in good shape physically, to be prepared for conception and pregnancy.

With Christmas approaching, the prime time for partying and fun may be just around the corner, but carrying a baby can place physical demands on the carrier’s body, so be sure to keep in mind your overall health as we enter into the party season. 

It might be worth scheduling a preconception visit with your OB/GYN to discuss your plans, as well as ensuring you are in good health to carry a pregnancy to full term. They may advise you about limiting alcohol and smoking as these can impact both male and female fertility. This is worth remembering during social gatherings with family and friends. 

Lifestyle factors can play a huge role in fertility, including sperm health and therefore the chances of successful conception, so try to make positive lifestyle changes today that will support you in your pregnancy plans. This includes regular exercise, getting enough quality sleep, and eating a nutritious diet full of fruits and vegetables and that is low in saturated fats and red meat.

2. Check your fertility health by scheduling sperm testing.

Unfortunately, over 14% of couples will struggle with infertility. Male fertility issues account for as much as 30–50% of infertility cases and so a good starting point when trying to conceive is to pre-test your semen to see if there are any issues you can start managing sooner rather than later. 

Semen analysis is a simple, non-invasive test that will allow you to move forward with your fertility plans, with increased knowledge and awareness of what you can do to improve your chances of pregnancy. To learn more about sperm testing, you can read all about our easy at-home sperm analysis kit.

3. Start taking supplements to boost your fertility.

To successfully conceive and carry a baby to term, both partners need a wide range of nutrients in their diet to ensure their bodies are functioning optimally. Sometimes, getting all the nutrients you need through your diet alone can be daunting. Many clinicians recommend supplementing your diet with specific antioxidants and minerals to boost your fertility. This can help to reduce oxidative stress, which can negatively impact fertility. If you’re not sure exactly which supplements will benefit you personally, you can read all about fertility-boosting supplements here, or to make everything easier, Legacy has a male fertility supplement that contains everything you might need to boost your sperm health.

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