The science behind sex selection

If you're trying for a baby, you may be wondering if it's possible to choose whether you have a baby boy or baby girl. It's only possible for parents to choose the sex of their baby in the UK if there's a medical reason for it. If you were hoping to balance your family with the opposite sex to your first child, or if you’d always imagined having a little boy or a little girl, there are still methods you can try at home. Learn more about sex selection and the Shettles method, Whelan method and Billings method.

Can we choose the sex of our child?

Fertility specialists have the ability to create and identify embryos of either sex. However, it is illegal to choose your baby’s sex in the UK, unless there’s a medical reason for it (HFEA 2019).

You can only legally choose your baby’s sex in the UK if you have a serious genetic condition that you risk passing on to your children and only affects one of the two sexes (HFEA 2019). Only pre-implantation genetic diagnosis (PGD) has been approved as a technique which does this. So you can’t medically intervene with conception just to balance your family, or for social or cultural reasons (HFEA 2017, HFEA 2019).

There are some cheap, safe techniques that you can try in the privacy of your own home, but they work no better than leaving things to chance (FPA 2018, Pfeifer et al 2017, Tiberi et al 2018). Some couples choose to experiment with sex-selection kits or try old wives tales, such as eating more meat to conceive a boy.

What would qualify us for sex selection?

You’ll only qualify for PGD if you have a serious genetic disorder such as haemophilia or cystic fibrosis. PGD reduces the chance of your baby suffering from the same condition, partly by testing for the genes which carry it.

If you think you are eligible for PGD, your case will be decided by the Human Fertilisation and Embryology Authority (HFEA).

PGD is an in vitro fertilisation (IVF) technique. At the fertility clinic, embryos are created from your eggs and your partner’s sperm. After these are screened, a specialist implants one or maybe two embryos in your womb (uterus).

Sex, as well as genes, can play a part. The condition, Duchenne muscular dystrophy, for example, usually only affects boys (NHS 2018a). A disease such as haemophilia almost always passes from mum to son (NHS 2020). In these circumstances, your doctor would just use a female embryo. They wouldn’t need to test for specific genes as well.

Your clinic will probably offer to freeze any spare embryos. You can then use these if you suffer the distress of a miscarriage, or if you’d like to have more children later on. These days, using frozen embryos is at least as reliable as using fresh embryos. It is also less invasive, and cheaper than another cycle of IVF (Bosch et al 2020, Shapiro et al 2014).

PGD is expensive, and the NHS has a limit on the number of couples that it can fund. If you have it done privately, the cost is variable depending on which provider you choose. Each IVF cycle can cost an average of £2,000 to £5,000 or more, and this doesn’t include the bills for consultations, tests and drugs (NHS 2018b). PGD costs even more than that – around £10,000 per cycle, plus extras (Guy’s and St Thomas’ Private Healthcare 2018).

Not all fertility clinics in the UK are licensed to carry out PGD. The HFEA clinic search tool can help you to find a clinic that is.

What is sperm sorting?

This is when doctors try to separate male from female sperm. Sperm sorting for sex selection is illegal in the UK, but the service is offered, for a considerable fee, in some other countries (HFEA 2019).

The most-used technique is flow cytometry. Its accuracy is thought to be about 93 per cent for girls born using this method, and 85 percent for boys (Karabinus 2014).

Flow cytometry works by adding a fluorescent dye to your partner’s sperm sample. Experts can then tell male from female by seeing how the dye binds to the genetic material in the sperm.

Once sorted, you can then be inseminated with the chosen sperm using relatively cheap intrauterine insemination (IUI).

There’s no guarantee of success with sperm-sorting (HFEA 2019), and IUI is not as effective as in vitro fertilisation (IVF) (HFEA 2017).

Can timing sex decide if I'm having a boy or a girl?

There are two theories about whether this is possible: the Shettles method and the Whelan method, both named after the people who devised them. There's a third method which combines timing theories with observations of your cervical mucus – the Billings method. Bear in mind that these methods are theories rather than proven techniques. There’s no evidence that any of them work.

You may decide to try one method if you understand your cycle well enough. But bear in mind that, whichever method you try, you always have approximately a 50:50 chance of conceiving a baby of the sex you’d like.

The Shettles method

Dr Landrum Shettles developed this theory in the USA in the 1960s. It's based on the idea that male sperm move faster, but don’t live as long as female sperm. So if you want a boy, you should aim to have sex as close as possible to ovulation (Shettles and Rorvik 2006), as the boy sperm will beat the girl sperm in the race to the egg. If you want a girl, you need to have sex two days to four days before you ovulate. Use our ovulation calculator to find out when you’re likely to be ovulating.

Shettles claimed that it’s 75 per cent effective for girls and 80 per cent effective for boys (Shettles and Rorvik 2006).

What is the Whelan method?

The Whelan method contradicts the Shettles method. It suggests that biochemical changes earlier on in your cycle favour boy-producing sperm. So if you want a boy, you should have sex four days to six days before you ovulate. If you want a girl you should have sex two days to three days before you ovulate (Whelan 1991).

Whelan claimed her method is 68 per cent effective for boys and 56 per cent effective for girls (Whelan 1991).

What is the Billings method?

The Billings method complements the Shettles method. It says you should time having sex around the look and feel of your cervical mucus.

For a girl, the idea is that you have sex at what they call the 'pre-peak stage' before ovulation (thicker and stickier), and then don't have sex again for that cycle. For a boy, you should wait for the 'post-peak stage' closer to ovulation (thin and clear). An African study of the Billings method reported an astonishing 95 per cent success rate (McSweeney 2011).

How about sex selection kits?

DIY kits that claim to help with sex selection are based on the Shettles theory. Separate girl and boy kits cost about £100. They include instructions, a basal thermometer, ovulation predictor test sticks, supplement tablets, and douches that are supposedly sex-specific.

Not surprisingly, the manufacturers commonly claim success rates of over 90 per cent. But there's no reliable scientific evidence to back up these claims.

• Find out more about how to detect ovulation. 
• Taking longer than you expected to get pregnant? Learn the most common causes of infertility.
• What you need to know about in vitro fertilisation (IVF).