This is an interesting finding - it has long been assumed that male fetuses are more vulnerable than females, with more males being conceived, but the margin narrows by the end of nine months. From New Scientist.
Male fetuses ignore their stressed-out mothers
- 15:00 20 July 2010 by Wendy Zukerman
Male fetuses ignore their mothers' response to stress – unlike females, which are very sensitive to it. The finding could lead to better treatments for male fetuses at risk of premature birth.
It is known that when a pregnant woman produces the stress hormone cortisol, it can cross the placenta. But it has been unclear how this affects fetal development, and whether female and male fetuses respond differently to the hormone.
During an asthma attack, high levels of cortisol are released. So Vicki Clifton and colleagues at the University of Adelaide in South Australia investigated the effect of cortisol on fetuses by following 123 asthmatic women and 51 healthy women during their pregnancies, recording the severity of each woman's asthma and her medication at 12, 18 and 30 weeks of pregnancy.
Forty-five minutes after the women gave birth, Clifton and her team measured the cortisol in their umbilical cord blood and analysed the placenta for the expression of genes related to stress response. She also recorded the newborn's sex and birth weight.
Stressful information
Baby girls born to women with moderate to severe asthma had higher levels of cortisol in their cord blood – an average of 245 millimoles per litre – compared with girls born to controls and mildly asthmatic women, who averaged 202 and 209 millimoles per litre respectively.
However, no difference in cortisol levels was observed in baby boys born to either group.
The team also observed that 22.5 per cent of girls born to asthmatic women were small for their gestational age, meaning they were among the lightest 10 per cent of all babies born worldwide. But just 9.5 per cent of girls born to healthy women fell into this group. Again, no difference was observed in boys from either group.
"Females are very sensitive to what's happening in mum's body, but males just ignore it," says Clifton, who presented her results at the University of Adelaide last week.
Small strategy
Low birth weight is associated with hypertension, diabetes and depression in adulthood, but a smaller fetus copes better with adversity in the womb, such as a drop in nutrients during an asthma attack. But the males take a gamble, says Clifton, ignoring the rise in cortisol and continuing to grow at a regular pace.
"There must be some benefit in males being bigger at birth, and this is worth the risk [of being affected by an associated drop in nutrients]," says Tim Moss, a prenatal physiologist at Monash University in Melbourne, Australia.
Moss says the work has important clinical applications that "could help us to reduce the vulnerability of male infants".
For example, in obstetric practice, stress hormones are routinely administered to women who are at risk of preterm delivery to induce faster maturation of the fetus. The treatment seems less effective in males than females – this study could explain why, and lead to new methods of aiding male development.
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