THE TIMES – 30 March 1995
Mother’s battle of the bulge
Dr James Le Fanu challenges a biologist’s claims that conflict rather than harmony reigns in the womb.
Thanks to ultrasound the development of a foetus in its mother’s womb can be observed virtually from the beginning, and it has become clear that the foetus is not just growing, but orchestrating the physiological changes in its mother that allow it to do so.
This process starts right from the moment of fertilisation. Then the conceptus, still little more than a few undifferentiated cells, secretes the hormone HCG which maintains the secretion of progesterone by the ovary. This prevents menstruation, so the lining of the womb, or endometrium, is kept ready for implantation. As soon as implantation occurs the embryo secretes another hormone which encourages the endometrium to become richer with the nutrients and blood vessels necessary for its growth.
The foetal placenta which attaches itself to the wall of the uterus is the means through which the foetus will obtain all the energy and nutrients it requires. But the placenta is much more than a communication channel, it is a factory producing enormous quantities of hormones which pass into the mother’s circulation.
The most important of these is oestrogen, which encourages the uterus to grow to a capacity 1,000 times greater than in the non-pregnant state, stimulates the milk-producing cells of the breast, and increases the amount of blood circulating in the veins and arteries by 50 per cent.
Then there is human placental lactogen, or HPL, which mobilises free fatty acids from the mother’s fat stores to provide the energy for the foetus to grow, and increases the amount of insulin secreted from the mother’s pancreas. This in turn boosts amino acids, the building blocks of foetal tissues.
The precise details of these and many other foetal influences on the mother fill chapters of modern textbooks of obstetrics, but the general impression is of a mysterious symbiotic relationship, in which mother and foetus conspire to realise the outcome that is so clearly desirable for both the birth of a normal healthy baby.
Next Monday a contrary view will be proposed in a Horizon documentary (BBC2, 8pm) devoted to Harvard biologist David Haig. Rather than a harmonious relationship between mother and foetus, Dr Haig sees conflict a struggle between the interests of the genes of the foetus and those of the mother. “Foetal genes are selected to increase the transfer of nutrients to the foetus, and maternal genes are selected to limit that transfer,” Dr Haig writes in The Quarterly Review of Biology. “Thus a process of evolutionary escalation is predicted in which foetal actions are opposed by maternal countermeasures.” Dr Haig elaborates his theory to explain the two main complications of pregnancy diabetes and raised blood pressure. Influenced by HPL from the placenta, the maternal metabolism of glucose increases during pregnancy to provide the energy for the foetus to grow. This in turn increases the mother’s requirements for insulin to keep the glucose levels in her own blood within normal limits. If the insulin-producing cells in the pancreas are unable to deliver, her blood sugar rises, resulting in diabetes.
Similarly the maternal blood pressure influences the perfusion of the placenta, and Dr Haig postulates that the foetal cells, to improve their chances of survival, secrete a substance to boost the mother’s blood pressure. This may lead to the condition of pre-eclampsia which damages the mother’s kidneys and can, if untreated, result in convulsions and death.
This evidence of conflict might just as readily be explained as an unfortunate failure of the complex homeostatic adjustments the mother has to make to protect the survival of the baby. Indeed, it is difficult to see how the foetal genes are pursuing their self-interest by generating conflict that results in conditions which, before modern medicine, were likely to lead to its own death, if not its mother’s.
But Dr Haig’s theory does highlight the most remarkable phenomenon in human reproductive biology. Theoretically the mother should reject the foetus because it is genetically different, just as she would reject a transplanted organ. She does not do so, however, because the cells of the placenta are unique in not expressing on their surface the antigens which the mother’s immune system would interpret as foreign. The foetus is immunologically “invisible”.
In evolutionary terms this could be interpreted as evidence of a successful adaptation by which the "selfish" genes of the foetus ensure their own continued propagation but it might more appropriately be described as a miracle.
Copyright: Times Newspapers Ltd