One of our biggest fears about designer babies is totally wrong

An oft-cited concern about the potential unintended consequences of designing babies according to parents' preferences is that more couples will choose to have boys than girls.

It's not an unreasonable concern. In India and China, two countries where sons traditionally are preferred over daughters, years of abortions based on sex have severely distorted the ratio of men to women.

But that concern may not be universally applicable. Prospective parents who try to conceive through in vitro fertilization (IVF) at some fertility clinics in the United States can already choose the sex of their children - and there's not evidence baby girls will become rarer.

IVF is a procedure in which eggs are taken from a woman's ovaries and fertilized with sperm in a petri dish. Then one or more fertilized embryos are placed in the woman's uterus, where one will hopefully implant and grow into a baby.

If parents want to have a baby of a particular sex, they can add a procedure called preimplantation genetic screening (PGS) to an IVF cycle. The procedure allows them to see the sex of all the embryos created for them, and they can choose to have only male or female embryos transferred to the woman.

In the US, one of only a few countries where PGS to choose the sex of children without a medical reason is legal, many people are actually doing the procedure to make sure they have baby girls, not boys.

However, there isn't comprehensive data about how many couples try to use PGS to ensure they have a child with their preferred sex. Reporting for the Wall Street Journal, Sumathi Reddy got conflicting accounts from fertility clinics about whether selecting the sex of children based on personal preference is becoming more common.

The practice is not likely to become widespread anyway. IVF is an intense medical procedure, and it's expensive, too. Getting PGS in addition to IVF can cost $15,000 to $20,000 total per attempt, Reddy writes, thousands of dollars more than IVF alone. IVF also does not have a 100% success rate, and since many insurance plans don't cover it, couples would end up paying a hefty price tag for a baby of their preferred sex (though some say it's worth it).

Even if many were willing to go through the procedure and could afford it, that doesn't necessarily mean the ratio of boys to girls would get skewed. A study published in Fertility and Sterility found that 50% of the 1,197 Americans surveyed said they would prefer having a family with an equal number of boys and girls (another 27% said they had no preference).

Those answers suggested that - if they were so inclined - most Americans might use PGS and IVF to preserve an even sex ratio, not distort it.