How do doctors determine a baby’s gender before birth?

Cells present in the amniotic fluid within the fetal sac are largely of fetal origin, and they contain the secret of a baby’s sex.

In the procedure called amniocentesis, a sample of this fluid is examined and the baby’s sex revealed. Because of a minimal but nevertheless real risk to the health of the fetus and the mother, however, and because of controversial moral questions about abortion for sex choice, few doctors will perform amniocentesis for the purpose of sex identification alone.

There are numerous instances, though, when taking a sample of amniotic fluid for sex determination is justifiable. These include cases of mothers who are carriers of an X chromosome linked disorder such as hemophilia, or Duchenne’s dystrophy, which can be transmitted only to a male child.

Amniocentesis for genetic purposes, including sex determination, is usually conducted during the sixteenth to eighteenth weeks of pregnancy, calculated from the first day of the mother’s last menstrual period.

At this time there is enough fetal development to obtain accurate results by chromosome analysis, yet the pregnancy could still legally and safely be terminated should that be desired. First sonography is performed: sound waves are sent through the woman’s abdominal wall, reflected back by the intrauterine structures, and translated into shapes on a screen. Those shapes reveal the position of the placenta and of the baby’s head and body, thus providing information about the baby’s health.

Next a 4 inch needle is inserted through the mother’s skin and uterine wall, into the amniotic cavity. At some centers a local anesthetic is administered. Every effort is made to insert the needle into the best pocket of fluid, as far away from fetal structures as possible; sonography is sometimes used to guide the physician here as well. A sample of fluid is then withdrawn for analysis. Sonography and the fluid collection procedure take about an hour.

Fetal cells obtained by amniocentesis, which derive mostly from the skin but may come from the respiratory or gastrointestinal tract, umbilical cord, and so on, are separated from the surrounding fluid.

In order to obtain a substantial number on which to base an analysis, laboratories culture the fetal cells in a medium containing vitamins, minerals, and antibiotics. There, the cells multiply by dividing in half for a period of about three weeks, at which point a chemical is added to the culture to stop the process at a time in cell division when the chromosomes are visible.

Then a sample of the cells is stained and examined under powerful magnification; twenty to twenty five cells are analyzed in detail, and several are photographed. The structure of the chromosomes, made visible by the stain, indicates their type, which in turn reveals the baby’s sex.

Chromosomes are elongated structures in the cell nucleus, which contain DNA, the genes that determine hereditary factors. The number of chromosomes is constant for each species; humans have forty six, for example, but fruit flies have only eight.

Chromosomes appear in each cell in pairs, one member of the pair deriving from the egg, or ovum, the other from the sperm. Among our chromosomes are two that determine sex. Females have a pair of equivalent chromosomes, XX, and males have a pair unequal in size, XY, the Y being smaller. Before fertilization, egg and sperm cells both undergo nuclear division, which reduces the number of chromosomes in each cell by half, thus opening the way for linkage with each other.

After this “reduction division,” each egg cell contains one X chromosome, and each sperm carries either an X or a Y chromosome. If the egg is fertilized by a sperm carrying an X chromosome, the offspring will be female (XX); if a sperm bearing a Y chromosome fertilizes the egg, it will develop into a male (XY).

Chromosome analysis through amniocentesis is therefore a reliable 99 percent accurate, means of discovering a baby’s sex before birth, two identical chromosomes indicating the presence of a female fetus, two different chromosomes indicating that a male is on the way.