Am I having a boy or girl?

You're bound to be curious about your baby's sex, even if you're waiting until the birth to find out.

But what if you could tell just by how warm the weather was when you conceived, or by how bad your morning sickness was? Folk tales are one thing, but which real-life factors may affect the sex of your baby?

Most of the time, mother nature is very good at balancing the number of boys born with the number of girls. If there are too many boys, then over a few generations more girls will be born until the numbers are more balanced (Gellatly 2019).

A word of caution! Changes to the number of boys and girls born takes time to happen and researchers often look at big groups, like the population of a country, to measure these changes.

So, the things that may make a difference to the sex of your unborn baby are likely to have only a very small effect. There's no guarantee that if any of these things happen, it'll dictate whether you end up with a girl or a boy!

Keep in mind, much of the scientific evidence here is slight, and some of it is hotly debated by experts. But it's still fun to try to guess. You may also want to read about more accurate ways to find out your baby's sex.

What could make it more likely that I'm having a baby boy?

In the UK, and across the world, there are more boys born than girls (UNPF 2017). For every 100 girls born, around 105 boys are born (UNPF 2017). So, in general you're slightly more likely to have a boy than a girl.

You may also be slightly more likely to have a boy if:

It was warm when your baby was conceived

In years when the weather is warmer, more boys tend to be born (Pavić 2015). Some research also shows that more boys might be born in warmer climates (Pavić 2015).

But before you pack your bags and head to the Caribbean, other research shows that some tropical countries have a greater number of girls than boys (Navara 2009).

A worldwide survey showed that more girls are born in the tropics (the region around the equator) than anywhere else in the world (Navara 2009).

You and your partner are living together

One large study of 86,000 parents showed that couples who were living together when they conceived were slightly more likely to have a boy (Norberg 2004).

But the slight increase really is tiny! Just over half (51.5 per cent) of couples living together before conception had a boy, compared with 49.9 per cent of those who lived apart. Why living together results in more boys is not entirely clear.

Your baby was conceived following IVF or IUI

Conceiving after having in vitro fertilisation (IVF) or intrauterine insemination (IUI) makes it more likely you'll have a boy (Maalouf et al 2014). The time when you have the embryo transferred during IVF may also make a difference. The later the transfer's done, the more likely you are to have a boy (Maalouf et al 2014).

Doctors don't know exactly why this is, but it may be because male embryos develop faster in the laboratory than females, so the male embryos are more likely to be used (Maalouf et al 2014). It may also be down to which sperm were used during IVF and IUI. Sperm carrying the Y chromosome may also be more likely to be chosen to fertilise the egg (Maalouf et al 2014).

Your baby’s dad has lots of brothers

One research study looked more than 900 family trees and discovered that men with lots of brothers are more likely to have sons, whereas men with lots of sisters are more likely to have daughters (Gellatly 2009). It's thought that men inherit a tendency to have more sons or more daughters from their parents (Gellatly 2009).

Your baby's dad determines the sex of your baby, depending on whether their sperm is carrying an X or Y chromosome. All your eggs carry an X chromosome. So:

• An X chromosome sperm joins with your X chromosome egg to make a baby girl (XX).
• A Y chromosome sperm joins with your X chromosome egg to make a baby boy (XY).

The research study suggests that a special gene controls whether a man's sperm contains more X or more Y chromosomes, and that affects the sex of his children (Gellatly 2009).

What could make it more likely I'm having a baby girl?

You may be slightly more likely to conceive a girl if:

The dad-to-be is over 35 years old

The older the dad, the less likely you are to have a boy and the more likely you are to have a girl (Jacobsen 1999).

Doctors don't know exactly why this happens. But it may be because as men get older, they tend to have sex less often or because their hormone levels change (Jacobsen 1999). This could affect sperm and make it more likely that a female sperm will fertilise the egg (Jacobsen 1999).

You’ve been really stressed around the time you conceived

Quite a few research studies have shown that during times when women are under high levels of stress, fewer boys are born (Bae et al 2017). This could be after war, natural disasters, extreme weather events, and times of huge political change (Bae et al 2017).

Stress hormones, such as cortisol, can affect your ability to conceive and how your pregnancy goes (Bae et al 2017). One small piece of research showed that women who were making lots of stress hormones were less likely to have a boy (Chason et al 2012).

Another piece of research explored whether your job could affect whether you have a boy or a girl. That showed that women in high-stress jobs, including full-time mums at home, were more likely to have a girl (Ruckstuhl et al 2010).

You have severe pregnancy sickness

If you have severe pregnancy sickness (hyperemesis gravidarum), you're slightly more likely to have a baby girl (Veenendaal et al 2011).

Researchers looked at the results from 13 studies and found that 55 per cent of babies born to women who'd had severe pregnancy sickness were girls (Veenendaal et al 2011). This compared to 49 per cent in a group of women who didn't have severe morning sickness (Veenendaal et al 201).

This might be due to high levels of a hormone called human chorionic gonadotrophin or hCG (Veenendaal et al 2011). The levels of hCG in your body are higher when you’re having a girl (or twins) and high levels are linked with pregnancy sickness too (Veenendaal et al 2011).

Your baby's dad has been exposed to pollution

A few small research studies have looked at the effects of different kinds of pollution on sperm. Air pollution may affect sperm so that men produce more sperm that carry the female X chromosome, compared to the male Y chromosome (Radwan 2018). This could mean you're more likely to have a girl than a boy if you live in a polluted area.

Another piece of research showed that substances called enzyme-disrupting chemicals can also affect the number of male sperm a man produces (Song et al 2018). These substances, which include things like pesticides and chemicals added to personal-care products, may affect hormones in the body (WHO nd). If the dad-to-be has been exposed to these kinds of chemicals, you may be slightly more likely to have a girl (Song et al 2018).

You conceived after having ICSI

Getting pregnant after intracytoplasmic sperm injection (ICSI) makes it slightly more likely that you'll have a girl (Maalouf et al 2014).

The reason why this happens may be linked to the sperm that are used. In IVF and IUI the way the sperm meets the egg is closest to the way it happens naturally, where the sperm compete to reach the egg (Maalouf et al 2014).

In ICSI, the sperm are specifically chosen (Maalouf et al 2014). It's most likely that you're having ICSI because of difficulties with sperm, and the sperm that have the X female chromosome are most likely to be chosen during the procedure because they're in the best condition (Maalouf et al 2014).

Take our boy or girl quiz

Find out whether you're more likely to have a girl or a boy.