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How to tell if you're having a girl or a boy

Are you having a boy or a girl? You've probably been curious since the moment you found out you were pregnant. From old wives' tales to scientific studies and Mumsnetters' own experiences, here's a guide to the most plausible – and some downright implausible but still good fun – indicators of what your baby's sex will be…

By Mumsnet HQ | Last updated Jul 13, 2021

Ultrasound of a baby

At what scan do you find out the baby's sex?

The dark ages of not knowing until birth whether you're having a baby girl or baby boy are long gone – but until you find out, the suspense can be almost unbearable.

Your baby's sex is determined at the moment of conception, when the sperm meets the egg and the baby gets its 23 chromosomes from each parent. The genitals start to develop at around 11 weeks but you won't be able to find out the baby's sex at your 12 week scan, as things won't be sufficiently developed by then.

Instead, it will be another several weeks before your doctor can determine your baby's sex from an ultrasound. Your 20 week scan is the big one when all will be revealed (although some couples still like to sustain the mystery all the way to the baby's birth).

Myths about determining your unborn baby's sex

Every pregnant woman and her partner are bound to be curious about their baby's sex. For this reason, myths have abounded down the ages about the signs to look for to see if you're having a boy or a girl:

  • Heart-rate. If your baby’s heart-rate is under 140 beats per minute then you are, according to myth, having a boy. Over 140 per minute and you should expect a girl. But the scientific basis for this theory is minimal.

  • Cravings. If you’re craving sweet things, it used to be said, you’re having a girl. Craving savoury snacks, on the other hand, was a sure sign you were having a boy. But what about those women find themselves wanting to eat coal or chalk?

  • Skull theory. Look at the ultrasound. Does your baby have a square jaw? Then you’re probably have a boy. Rounded jaw? Well, then obviously it’s a girl.

  • Morning sickness. If you’re having a boy, people used to claim, you’ll hardly suffer from morning sickness. If you’re having a girl, you’ll have a terrible time and wake up every morning with nausea. This is because girls’ hormones levels are higher.

  • Skin test. Does your skin feel soft? Then you’re having a girl. A bit dry? Expect a boy.

  • Baking powder test. Put two tablespoons of baking powder into a glass – preferably an old one that you don’t drink from any more. Urinate into the glass. If the liquid fizzes like lemonade then you’re having a girl. On second thoughts, don’t do this, as it is clearly nonsense and unhygienic.

These are just six of the best (or worst) myths that used to be told to pregnant women who were anxious to know their baby’s sex. There are many more but the truth is that you will have to be patient and wait for your 20-week scan.

In the meantime, here are some small indicators that you might find useful (just don’t blame us if they’re wrong):

Bump shape depends on your stomach muscles and innards – nothing else! It’s not an indicator of your baby’s sex.

Signs that you’re having a boy

  • Season of conception. If you conceived in summer then, according to some studies, you’re more likely to have a boy. One reason for this might be that minor illnesses – colds and flu etc – are more common in winter and the more fragile male embryo struggles to survive them. Girls tend to be tougher and can see off bacteria and viruses.

  • Parents are living together. This is a curious one but a large American study found that, if the parents are living together at the time of conception, you’re marginally more likely to have a boy.

  • Calorific diet. Women who eat more calories increase their chances of having a boy. Fewer calories and the male fetus is less likely to survive. Female fetuses are more resilient and have a better chance of surviving when the mother eats less.

  • Gestational diabetes. Some women develop gestational diabetes in pregnancy if they have high blood sugar levels. This can also be a sign that you’re carrying a boy.

Pregnant woman lying down

Signs that you’re having a girl

  • Your age. Older men produce fewer male sperm and women experience hormonal changes as they age. If you’re both over-35 then you are statistically more like to have a girl.

  • Weather. If you live somewhere that has extreme swings in the climate, like New York, say, where it’s hot in the summer and freezing in winter, then you’re more likely to have a girl (although, of course, plenty of boys are born in such climates.). Again, it’s due to female fetuses' greater strength: they can handle the upheaval of extreme weather whereas for boy it can be too much. More girl babies are born in warm places and the tropics produces more than anywhere else in the world.

  • Your job. If you or your partner has a stressful job then you might be having a girl. This theory operates similarly to the weather one (see above): if you have a high-pressure job then some of the stress could be passed on to your baby. And guess what – girls are more likely to survive the stress. A survey of 16,000 pregnancies showed 56% of women with jobs considered stressful having a girl.

  • You’re overdue. If you have your baby past your due date then the chances of you having a girl are slightly increased.

Signs that you’re having twins

Have you considered that you might be having more than one baby? Multiple births are on the rise, with an increase in the UK of around 50% between 1984 and 2004. That still means a mere one percent of pregnancies are multiple births (98.5 percent of multiple births are twins). Here are a few signs that you might be carrying more than one baby:

  • Twins run in families. There’s a gene that makes a woman more likely to release two or more eggs during ovulation. So if there’s a family history of twins then it’s possible more might be on the way. Curiously, this only applies to non-identical twins.

  • A bigger bump. The science is by no means exact and you’re bound to think your bump is big, so this one is by no means definitive. But if you think you’re bump is bigger than other pregnant women’s bumps, especially at an early stage of pregnancy, then you might be having twins.

  • Morning sickness. As mentioned already, this used to be interpreted as a sign you were having a girl. But it’s also often taken as an indication that twins are on the way. The theory is that, if you’re having twins, you’ll experience even more hormonal changes, so your symptoms will be more acute.

Entertaining as all this speculation might be, the only way to know what gender your baby will be or, how many babies you’re having, is to see the ultrasound.

I'm 36 weeks pregnant with twins. I wasn't sick at all but was absolutely exhausted in the early weeks.

What Mumsnetters say

“The heartbeat theory worked for me. My sons both had heartbeats at 120-139 beats per minute. For my girls, it was 140-160.”

“I'm convinced I'm having a boy and my partner is convinced too. Maybe that’s because I already have a son. My friends say they can see me with a girl, but I absolutely cannot imagine it!”

“You should believe any of the signs about your baby’s sex. I had all the signs that my baby was going to be a boy, so I was convinced of it up until the moment she popped out.”

“My twin pregnancy was easier than my single pregnancy, but I had no clue whatsoever that I was having twins until I saw the two little bubbles on the ultrasound screen. My exact words were: “Tell me that’s not two. Oh blimey, it is isn’t it?”

“I have two sets of twins in my family and one set of twins in my DP family. I am petrified by the idea of having twins. But I also think it would save going through pregnancy again in a couple of years!”