A number of companies are now offering embryo screening to reduce disease risk and promote desirable traits such as intelligence, eye colour, or height. It feels futuristic, but it’s now. Venture-capital money has poured in; the founder of one company, Orchid, has been interviewed by the New York Times (Elon Musk is reportedly a customer); another company, Herasight, has been featured on CBS News; and a third company, Nucleus Genomics, is embroiled in a plagiarism scandal after launching an aggressive marketing campaign.

How does this new reproductive technology work? Is it scientifically sound? What does it mean for fairness, health, and our values concerning children? In what follows, I will discuss the process of embryo selection, its history, and—briefly—the ethics. The essay is quite long but each section is signposted. If less is more for you, skip over the sections that don’t interest you. You may think a history section sounds dull. It’s not. So please don’t feel tempted to skip that one. Let’s go.

I. The Process of Embryo Selection (in Brief)

Let’s imagine that you and your husband, Hugo, are planning a family. You have an uncle with schizophrenia and Hugo’s sister struggled throughout high school; you want your baby to grow up to be a healthy high-school graduate. You know that genes influence both of these outcomes based on fifty years of twin studies. So you opt for embryo screening and set about choosing a company from a marketplace that includes LifeView, Herasight, Nucleus, and Orchid Biosciences. (These are the ones I have found; there may be others.)

Next, you will choose your in vitro fertilisation (IVF) provider, a decision that can be taken in consultation with the embryo-selection company. Your IVF provider will then stimulate your follicles and harvest at least fifteen eggs (if you want to develop a larger number of embryos, you may want to be hyperstimulated). This is accomplished with daily hormone injections for eight to fourteen days, during which you’ll be monitored by ultrasound or blood tests. The eggs are usually retrieved under sedation and then fertilised using sperm already provided by Hugo. If his sperm is looking a bit tired, a single sperm can be injected into each egg.

The fertilised eggs will then sit, hands folded, silently multiplying nonlinearly (day two, about four cells; day four, about sixteen cells; day five between seventy and 200 cells). Growth and development will be checked daily by embryologists; some clinics use AI to analyse time-lapse video of the multiplying cells for additional quality control. At each stage of this process, you can ask for predictions about various outcomes. Data accrued from forty years of IVF allow us to determine statistical likelihoods such as the number of eggs that may be retrieved given your age, how many of those are likely to be fertilised, how many of those will develop into a blastocyst, and so on.

A blastocyst is reached about five or six days after fertilisation when the cells have formed a hollow ball with an inner mass and outer layer. The larger outer layer (trophectoderm) will develop into the placenta, and it can spare five or ten of its seventy to 200 cells. You don’t want to mess with the inner cell mass—that will go on to become the foetus. Many IVF clinics offer screening at the blastocyst stage for chromosomal abnormalities and genetic disorders like cystic fibrosis or Huntington’s Disease (more about this in the not-to-be-skipped, very interesting history section).

The IVF clinic will now extract a sample of cells from each embryo and send this biopsy material to a third-party company while the embryos are frozen and stored at the IVF clinic. The third-party company will analyse the cells it receives and produce a raw DNA data file for your embryo-selection company, a process that typically takes between two and four weeks. The embryo-selection company will also want a blood sample from you and Hugo, analysis of which will augment analysis of your embryos.

Having probed the genetic data, your embryo-selection company will produce a report on each embryo with a genetic probability score for traits like height, heart health, diabetes risk, intelligence, and so on. You will rank your embryos in order of preference based on your understanding of the report (the embryo-selection company may provide you with assistance from a genetic counsellor). Finally, you return to the IVF company and tell them which of your embryos you would like them to unfreeze and implant.

If the transfer is successful and the pregnancy develops, you can tell Hugo to start knitting. If not, you can re-start the process with your second preferred embryo—and so on. That is a spare account of the process, but there’s a ton of relevant detail...