Written by Mollie McGuigan
If you’re embarking on the journey to parenthood you’ll be hoping for a healthy, happy baby at the end.
You might also have a preference for the sex of that baby. There are many claims about influencing the outcome, including certain sex positions increasing your chance of having a baby boy, but what’s actually true? We asked two gynecologists.
Put simply, the sperm determines whether you conceive a baby boy or a baby girl. Here’s the science: Eggs carry the female X sex chromosome, while sperm carry either the female X or male Y chromosome. If the egg is fertilized with the X chromosome sperm, you will conceive a girl. If the egg is fertilized with the Y chromosome sperm, you will conceive a boy. There are also different combinations of chromosomes that can result in the baby being intersex.
It’s important to note here that there’s a difference between sex and gender. Your sex (female or male) is what you’re assigned at birth and relates to your biologically defined characteristics. Gender is your personal perception of yourself (so woman, man, fluid or somewhere in between). For some people, their sex and gender align. For others, they don’t and that’s completely normal.
The sex ratio, that’s the ratio of female to male births, is slightly skewed male. According to a 2011 World Health Organisation Report, around 49 percent of births are baby girls while 51 percent are baby boys.
While the majority of people simply want a healthy, happy baby, some scientists have developed theories surrounding conception and sex which have led others to google “sex positions for a baby boy.” This can be traced back to American doctor Landrum B. Shettles.
In the 1960s, Shettles developed “The Shettles Method,” a set of protocols to use before and during sex, which he claimed would help couples influence the sex of their baby. His controversial book, Your Baby’s Sex: Now You Can Choose, was published in 1971.
His advice included the best sex positions to conceive a baby boy and theories on vaginal pH. It was based on the belief that male sperm are smaller, faster, and die quicker, while female sperm are larger, heavier, and live longer. Due to X chromosome and Y chromosome sperm’s different characteristics, he claimed couples could influence the sex of their baby by timing intercourse, choosing certain sexual positions, and making dietary changes.
However, numerous clinical trials have dispelled his claims, including his theories on sperm presentation. “You won’t find a single scientist or a qualified medical professional of repute who will endorse these methods,” says London-based gynecologist Dr. Nitu Bajekal.
“There is no science to it. When you have a [approximately] 50 percent chance (of conceiving a boy or girl), then you can say anything and statistically, you have a 50 percent chance of being correct. They don’t withstand the test of rigor.”
Dr. Bajekal points out that while male sperm have been shown to swim faster, “we’re talking about lab conditions, not in real life.”
Another theory put forward by Dr. Shettles was the idea that having sex at certain times could influence whether you have a baby boy or a baby girl.
Dr. Shettles claimed male sperm die quicker, so they should spend as little time as possible waiting for an egg to be released. His theory, therefore, goes that couples should have sex on the day of ovulation — or just after it — to give Y chromosome sperm the best chance of fertilizing the egg if you’re hoping to conceive a boy.
However, a 1995 clinical trial studied the menstrual cycles of 221 women who were trying to conceive and found “the timing of sexual intercourse in relation to ovulation has no influence on the sex of the baby.”
Dr. Shettles believed vaginal pH had a bearing on whether you conceived a boy or a girl, but like his other claims above, this has since been refuted by scientists.
The pH scale measures how acidic or alkaline a liquid is, so 7 is neutral, greater than 7 is alkaline, and less than 7 is acidic. A healthy vaginal pH level is around 4–4.5.
Dr. Shettles claimed female Y chromosome sperm live longer in the vagina’s acidic environment than the male X chromosome sperm, which prefer a more alkaline environment. To have a greater chance of conceiving a boy, Shettles advised creating a more alkaline vaginal environment (more on that below).
However, Dr. Tiffanny Jones, reproductive endocrinologist and infertility specialist is quick to dispel this: “[A baby’s] sex cannot be influenced by any of these parameters.”
Dr. Shettles claimed exactly that, believing that alkaline secretions from female orgasms created favorable conditions for the Y chromosome sperm to fertilize the egg.
However, studies have since found female orgasms have little effect on vaginal pH (which is good news, considering the orgasm gap sits at 30 percent between heterosexual partners).
Simply put, no, having sex in certain positions won’t increase your chances of conceiving a baby boy. Though it’s easy to see where this thinking comes from.
Dr. Shettles advised couples to have deep, penetrative sex if they wanted to conceive a boy, so the speedy Y chromosome sperm can get as close as possible to the alkaline-rich cervix and reach the egg first. Ever since, couples have been trying that approach in the hope of conceiving a baby boy.
Dr. Bajekal is quick to reiterate that there has been little scientific research to support Shettles’s claims. “And you only need one sperm out of the millions to make its way in,” she says.
We now know that there’s nothing you can do in the bedroom to influence whether you have a baby boy or a baby girl. Though there’s a slightly greater chance you’ll have a baby boy (due to the ratio of male to female births), sex positions, timing sex, and your vaginal pH have no bearing.
“When you have a lot of myths around fertility and conception, it harms vulnerable groups,” Dr. Bajekal says. “Stress is a huge factor in fertility, and the last thing you want is to bring in sex preference to add another layer of stress. It is so outdated to think about sex preference. Gender is fluid [after all] — the person might identify as non-binary.”
There are, however, things you can do to give conception a helping hand. Dr. Bajekal recommends:
- Looking after your physical and mental health
- Stopping smoking and drinking alcohol
- Maintaining a healthy weight
- Having regular sex
- Eating a diet that’s low in ultra-processed food and high in fruit, vegetables, whole beans, and grains
Knowing when your fertile window falls in your cycle makes all the difference when you’re trying to have a baby. Period tracking apps, like Flo, take away the guesswork by monitoring what’s going on in your body to let you know when you’re likely to be ovulating. You can then have sex (or elect for insemination) on these days.
Your doctor may also suggest that you take folic acid. It’s important that you start to build up your supplies of folic acid before conception because it can help prevent neural tube birth defects later down the line. Around 400 mcg is the recommended amount.
It’s common for conception to take a while. However, if pregnancy isn’t happening after 12 months of trying (if you’re under 35) or after six months (if you’re 35+), see your doctor or health care professional so they can offer help and support.