Tuesday, 6 April 2010

Gimme a pack of low birth weights

From the Daily Telegraph:

Women who exercise during pregnancy produce 'lighter babies'

As Tim Worstall points out, haven't we been told that low birth weights are a bad thing ever since they were associated with smoking in pregnancy? So are we now going to stop these women exercising?

Dr Paul Hofman, from the University of Auckland in New Zealand, said: ''Our findings show that regular aerobic exercise alters the maternal environment in some way that has an impact on nutrient stimulation of fetal growth, resulting in a reduction in offspring birth weight.

''Given that large birth size is associated with an increased risk of obesity, a modest reduction in birth weight may have long-term health benefits for offspring by lowering this risk in later life."

I can't say with any confidence whether low birth weights are good, bad or indifferent, but a little consistency from the medics would be nice. It's hard to avoid the conclusion that 'lighter babies' are good/bad depending on whether the researcher is involved in the crusade against smoking or the war against obesity.

And if larger babies really do grow up to be fatter, doesn't that suggest that some people are just born fat and that the so-called 'obesogenic environment' has got nothing to do with it?

Incidentally, I remember reading somewhere (can't find it now) that the weight difference between smokers' and nonsmokers' babies was ridiculously small—something like 9 grammes. Do any of my learned readers have the figures?


Tony Palazzolo said...

Here in America, they like big fat babies. They have to do a C section and the hospitals get more money.

Ben said...

Unfortunately, I don't have the links, but I do have some citations:
“… [Dr. Allen Wilcox] said, ‘the babies of smoking mothers had a higher survival rate.’ As he explained this paradoxical finding, although smoking interferes with weight gain, it does not shorten pregnancy. Thus, among smoking women the smaller babies are more likely to be born full-term, but the smaller babies born to nonsmoking mothers are more likely to be born prematurely. So, he deduced, it is their prematurity, not their low birthweight, that explains the higher infant mortality rate among babies of low birth weight who are born to nonsmokers,” (“High Infant Mortality in U.S. Is Linked to Premature Births,” Jane E. Brody, New York Times, Mar 1, l995).

"A significant reduction in birth weight was found to be associated with an average caffeine intake of more than or equal to 71 mg per day, after adjustment for gestational age, infant sex, parity, and maternal height and weight, but only in infants born to nonsmoking mothers." Vlajinac HD, Petrovic RR, Marinkovic JM, Sipetic SB, and Adanja BJ, “Effect of Caffeine Intake During Pregnancy on Birth Weight,” American Journal of Epidemiology, l997; 145:335-8.

“The deficits of weight at birth in children born to mothers who smoked during pregnancy are overcome by 6 months of age.” Conter V, Cortinovis I, Rogari P, Riva L, “Weight growth in infants born to mothers who smoked during pregnancy,” British Medical Journal, Mar 25 l995; 310(6982):768-771.

Fredrik Eich said...

I have not got a link to the study but there was this "Damage to unborn baby from smoking ‘negligible’ in the first five months" times online article

Anonymous said...

Actually, it's quite a bit more than 9g. It seems to be more like 150g, or around 5% of baby weight when controlling for other factors like maternal weight and SES (
Br Med J 1972;2:127-130
, BMJ 1989;298:795-801).

Though as Fred above says, there seems to be some evidence to indicate that stopping smoking in the first trimester or thereabouts reduces this effect by a significant degree.

Anonymous said...

I immediately thought of the paper to which Frdrik refers. I remember reading it and I remember the criticism Emma Tominey got from the anti tobacco industry. I even remember posting a comment to the effect that she was very brave or had a large private income which meant that gaining further employment wasn't crucial. It is a very interesting paper but doesn't answer your question as why big babies are now worse than small.

Fredrik Eich said...

"Offspring of exercisers were on average 143 ± 94 grams lighter than their control counterparts, however there was no difference in birth length."

So ~5%!

I would be very surprised if smoking status was not recorded. We shall have to wait for when it is published.