FOR us, oxygen is a benign gas essential to life, but for a developing fetus
it can be a killer. One in six pregnancies comes to a premature end, and oxygen
may play a key role in these miscarriages, according to scientists who have
measured levels of oxygen in the placenta through the first third of
pregnancy.
The results have encouraged senior obstetrician Lesley Regan of Imperial
College, London, to advocate testing whether high doses of antioxidant vitamins
could reduce the risk of miscarriage. One of the researchers, Graham Burton of
Cambridge University, says, 鈥淭here might well be women, not at high risk, who
have miscarriages for no known reason. This provides us with an
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Burton, with Eric Jauniaux of University College London, placed a tiny
monitor in the placentas of 30 women. They found that oxygen levels in blood
entering the umbilical chord tripled between the 8th and 15th weeks of
pregnancy. Because their technique might harm the fetus, the researchers only
studied women who had chosen to terminate normal pregnancies.
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Most embryologists have traditionally thought that oxygen levels rise gently
throughout pregnancy. The new results confirm the suspicions of others that
oxygen levels suddenly shoot up. 鈥淭his clearly shows us there are two distinct
phases in pregnancy,鈥� says Burton. 鈥淚n the first trimester in the placenta there
is a very, very low oxygen concentration.鈥�
As well as showing that the fetus requires very low oxygen for normal
development early in pregnancy, the results suggest that the change from low to
high oxygen levels is a relatively sudden and stressful event for the fetus. 鈥淎t
10 to 12 weeks, as oxygen comes in, this surge, combined with any additional
factors, may be enough to cause spontaneous miscarriage,鈥� says Burton.
Research by Burton and Jauniaux suggests that cells called cytotrophoblasts,
which anchor the placenta in the womb and invade the blood vessels to limit
oxygen supply to the fetus, dissipate at around 8 to 10 weeks. This would allow
more oxygen in. Taking antioxidant vitamins to mop up free-radical chemicals
formed by oxygen should help protect the fetus, says Jauniaux.
Regan agrees that there should now be trials of antioxidant vitamins. She has
pledged to work with Jauniaux on such a study. But she notes: 鈥淭here鈥檚 no
information in the literature to indicate that vitamins C and E are safe in
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