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David Bainbridge


The science of pregnancy

Book review by Anthony Campbell. Copyright © Anthony Campbell (2001).

This book traces the events of pregnancy from conception to the early postnatal period. It is mainly about human pregnancy but pregnancy in other species is used to account for various peculiarities of the process. The style is popular but a good many scientific facts are included, some quite surprising. It should appeal to young parents or parents-to-be with an interest in science and who wish to understand the details of the experience they are going through.

The book begins at the beginning, with a fairly detailed explanation of basic genetics and a discussion of why sex exists and why there is such a marked discrepancy in size between sperm and egg. Bainbridge inclines to the theory that this serves to prevent conflict between paternal and maternal mitochondria. (See Mark Ridley's Mendel's Demon for a fuller discussion of this idea.)

Once conception has occurred, the embryo has to inform the mother of the fact so that she switches into pregnancy to mode, which needs to happen quickly, within a couple of weeks at most. This question has been extensively studied in deer, in whom it apparently depends on release of interferon by the embryo. We normally think of interferon as an antiviral defence substance, so it is surprising to find it fulfilling this role in conception; the suggestion is that it originally evolved to prevent viral infection in the embryo and then, over millions of years, deer mothers evolved the ability to detect interferon release and to recognize it as a sign that conception had occurred. The interferon mechanism may also help the mother deer to decide whether to continue the pregnancy, depending on the sex of the offspring; the more dominant the mother, the more likely she is to produce male calves. Human embryos, however, don't produce interferon; it is probably chorionic gonadotrophin which acts as the signal here.

Chorionic gonadotrophin is probably also involved in the puzzling phenomenon of morning sickness. Bainbridge doesn't accept the theory that this has evolved as a protective mechanism, to prevent the mother from eating harmful substances during pregnancy, but prefers the view that chorionic gonadotrophin interacts in some way with the mother's thyroid control mechanism. He also reports the interesting discovery that children who cause severe morning sickness in their mothers are more likely to have a pronounced taste for salt after they are born.

Once pregnancy is established, the embryo has to go through an incredibly complex sequence of developmental stages to change from an undifferentiated ball of cells into a baby. Bainbridge makes a pretty good job of describing this process comprehensibly. The evolutionary background is emphasized throughout, and there is a good discussion of the shortcomings of Haeckel's theory that the embryo recapitulates the evolutionary story from fish through amphibian to mammal. Some diagrams are included in this section but probably most readers would have welcomed more of these.

Other subjects discussed include the means which the embryo uses to prevent its being rejected by the mother's immune system and the nature of the signal that starts the process of birth. In sheep it is the lamb that initiates delivery, but in humans, rather surprisingly, it is neither the mother nor the baby but the placenta, which secretes progressively larger amounts of a hormone normally found in the brain, called corticotrophin releasing hormone. The stopwatch, it appears, is set early in pregnancy, at the time of the formation of the placenta.

In his concluding section Bainbridge suggests, rather controversially, that this whole physiological process may before long be superseded by science, when it becomes possible to carry out the complete development of the fetus outside the body with an artificial placenta. This has already been achieved with goat fetuses in Japan and the kids survive for some months after "birth". We already have in-vitro fertilization and insemination by donor; soon we may have controllable conception, no childbirth, no breast-feeding, and no pregnancy. People will no doubt resist this at first, but will they still do so when it becomes safer than normal pregnancy and allows women without a uterus to have children of their own? Bainbridge thinks that they won't resist it, and it's difficult to believe that he is wrong about this.

%T A Visitor Within
%S The Science of Pregnancy
%A David Bainbridge
%I Weidenfeld and Nicolson
%C London
%D 2000
%G ISBN 0-297-6-4677-X
%P 292 pp
%K biology
%O Line illustrations
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