Early development

9 Selecting the sex of a child

Once a pregnancy has been established, many couples are anxious to know the sex of their unborn baby. The reasons for this are many, ranging from the prosaic (will the baby be able to use its brother's or sister's old clothes) to the deeply religious (as described for Hindus in Section 2). In many communities there is so much social pressure on mothers to produce the ‘right’ sex (usually male) that infanticide of the ‘wrong’ sex is widely practised. Because this is illegal in most societies, figures to substantiate this claim are not available. Although cultures differ in their regard for individual human life, it is probably true to say that no mother, whatever her beliefs, can be completely unmoved by the death of her baby. Although this matter is complicated by cultural factors, a lot of pain might be avoided if couples could choose the sex of their child.

Q Conduct a straw poll among your acquaintances, particularly any who are expecting a baby. Do they, or would they, want to know the sex of their unborn baby? What are their reasons for feeling this way? How many would like to be able to choose in advance the sex of their child?

A You probably found that many people do not mind what sex their baby is as long as it is healthy. However, some people have very strong feelings about this, one way or the other. This may be for cultural reasons, as we mentioned above. But it may also be because they have a family history of a genetic disease which affects only one sex, and therefore want a child of the other sex, which will be unaffected. Or it might simply be because they already have one or more children of a particular sex, and want some variety.

Q When does the sex of a baby become apparent?

A As you saw in Section 7, one of the first ‘markers’ of the new male individual is carried on the Y chromosome. This is first expressed at the 4-cell stage, so this is when embryos of different sexes can be functionally distinguished.

The sex of a fetus can be deternined using amniocentesis. This technique involves sampling the amniotic fluid (the liquid that surrounds the fetus): a hollow needle is inserted through the mother's abdomen and some of the amniotic fluid is withdrawn. In the fluid are found some fetal cells which have been shed during development. These can be tested for biochemical or chromosomal abnormalities, if the family history suggests that this is a possibility. The appearance of the chromosomes will also reveal the sex of the fetus. Amniocentesis is not routinely used, however, as it brings a risk of miscarriage.

As far as telling the sex of a fetus simply by looking, it is, of course, necessary to wait until the genitals are visible. Although the genitals are quite well developed by about 12 weeks’ gestation, they are very small, and hard to distinguish. In practice, it is often possible to tell the baby's sex by 17–18 weeks, at the time of a routine ultrasound scan. However, unless there is a good reason for telling the mother (such as to give her information about whether the baby is likely to suffer from an inherited disease affecting only one sex), many hospitals withhold this information on the grounds that some people would procure an abortion if the baby were found to be of the wrong sex. We leave you to think about the ethics of this.

How easy might it be to choose an embryo's sex? There are two approaches to this that are technically possible at present. One involves looking at the chromosomes of the fertilized egg or conceptus, to see if a Y chromosome is present or not. This is clearly possible only when the embryo is outside the mother's body at this stage.

Q When might this be the case?

A During an in vitro fertilization procedure (IVF).

In most IVF procedures, more embryos result than can realistically be replaced into the mother. The general rule is that no more than three embryos should be put back.

Q Why do you think this is?

A If all three should implant, this is a major physical load for a woman to cope with. Since there is only a certain amount of space in the uterus, too many embryos will mean that resources will be limited, and the babies will be small.

Excess embryos are usually frozen or discarded. However, since all the embryos are cultured in the laboratory for a short while to ensure that they are developing normally, it is a simple matter to use one cell of each to check the chromosomes. If embryos are sexed in this way, only those of the chosen sex can be replaced in the mother, ensuring that the baby will be the right sex. Loss of one cell is not a problem for the health of the embryo; as you saw above, at early stages the embryos are developmentally plastic, and one cell can easily replace another.

Embryo sexing is all very well in cases where the mother is undergoing IVF, but is clearly impractical for most cases.

Q Can you suggest another, easier technique for ensuring an embryo of a particular sex?

A Fertilizing an egg with either an X-bearing or a Y-bearing sperm. (Section 7).

This is fine in theory, but how can the two types of sperm be reliably separated? They are produced together, and the ejaculate contains a mixture of roughly equal amounts of each type. They look the same, are the same size, have the same proteins on their surface, and swim with the same random path.

Q Look back at Figure 8. Can you suggest a difference between X-bearing and Y-bearing sperm?

A The Y chromosome is smaller than the X chromosome. Therefore Y-bearing sperm contain less DNA than X-bearing sperm.

Because DNA accounts for a significant part of the weight of a sperm (remember there is very little cytoplasm), the difference in amount of DNA means that Y-bearing sperm are a little bit lighter than X-bearing sperm. Thus, you might expect them to be able to swim a little further or a little faster than X-bearing sperm, assuming the energy supply within the sperm is constant. This is difficult to demonstrate in practice. Because sperm swim in a completely random way, it needs sophisticated microscopes and measuring equipment to trace the paths taken by individual sperm after ejaculation. Furthermore, manipulating sperm so as to be able to measure them can interfere with their ability to fertilize.

There is one approach that has yielded some promising results. Although sperm move in a random way, it seems that they have a tendency to swim up a gradient of concentration (that is, from a dilute solution to a more concentrated solution) of some sugars. If ejaculated sperm are placed in a tube, on top of a carefully layered gradient of sugar, with a dilute solution at the top and a more concentrated solution at the bottom, then after a while the sperm can be shown to have swum down the tube (see Figure 18). It is claimed that those sperm that have swum the furthest are the lighter ones, that is, the Y-bearing sperm. The heavier, X-bearing sperm do not swim so far along the tube. In theory, taking sperm from the top or bottom of the tube would be expected to give pure populations of X- and Y-bearing sperm respectively. Based on this approach, many sperm-sexing centres have been established, which claim to be able to produce babies of the desired sex by artificial insemination with such ‘pure’ populations of sperm. Unfortunately, the success rate achieved by such methods is little different from the 50% that would be achieved with any pregnancy. This is due to the random paths taken by the sperm: although they have a net tendency to swim up a gradient, the tortuous path they take can negate any separating effect of the weight difference (which is less than 1%). So the most that can be obtained is two populations of sperm enriched in X- or Y-bearing members.

Figure 18, Gradient separation of X- and Y-bearing sperm.

In spite of the lack of certainty offered by sperm selection techniques, they enjoy a certain amount of popularity, particularly among people whose cultures strongly favour one sex of child. It seems clear that many people find this kind of intervention in their reproductive processes preferable to abortion or infanticide.