Doctors have known about the correlation between tobacco use and lower birth-weights for at least 20 years. And they think they know how it happens.

What is believed is that the nicotine acts as a vessel constrictor in the mother, causing decreased blood flow to the uterus during pregnancy. The result is less nutrition and oxygen for the baby, regardless of the mother’s intake of food.

That’s why it’s not surprising that all three categories of women in a study showed the same average weight gain during pregnancy and yet still showed significant differences in the sizes of their babies. The availability of the food (in the mother) is the same, but it’s not getting to the baby.

Smoking and mothering don’t mix. Babies of women who smoke during pregnancy are more than twice as likely to show signs of distress during delivery and to be born at low birth weights that put them at risk of mental retardation and other developmental disabilities than mothers who don’t smoke.

Those are two key findings gleaned from an analysis of more than 11,000 electronic birth records for babies born in 2009. Those records, obtained from the Department of Health, require that physicians note whether the baby’s mother used tobacco or alcohol during pregnancy.

The analysis compares three groups of women who had babies in 2009 – those who smoked, but had no other known medical risk; those who used alcohol, but had no other known medical risk; and those with no known medical risks.

Here are some of the study’s more interesting findings:

• All three categories of women – smokers, alcohol-users and those with no medical risks — gained an average of 31 to 32 pounds during pregnancy.
• Babies of smoking mothers were more than twice as likely to suffer from Meconium Aspiration Syndrome, a result of stress during labor and delivery that can lead to life-threatening pneumonia.
• Babies born to alcohol-using mothers were an average of 125 grams lighter than babies of no-medical risk moms.
• Babies of smoking mothers weighed an average of 250 grams less (about a half pound) than the babies of mothers who had no medical risks.

The higher percentage of low birth-weight babies born to smoking mothers (9 percent compared to 4 percent for non-smokers) reflects the current national rates of 11.5 percent for smoking mothers and 6.3 percent for non-smokers, according to the National Center for Health Statistics.

A lower birth-weight can lead to learning disabilities and other developmental problems in babies that may not be discovered until years later.

Alcohol use, especially early in pregnancy, can stunt the growth of the fetus and, in particular, brain development. Alcohol use during pregnancy is the third-leading cause of mental retardation in the United States.

The smaller brain size of alcohol-damaged babies may also explain why alcohol-using mothers in the study were only one-third as likely to have babies whose heads were bigger than their pelvis could deliver.

Cephalopelvic disproportion, when a baby’s head or body is too large to fit through the mother’s pelvis, occurred in 3.69 percent of no-risk mothers compared to 1.3 percent of alcohol-using mothers.

But women who drink during pregnancy pay a high price for the lower chance of a difficult delivery — the long-term effect on their baby/s brain.

Because even moderate alcohol consumption could possibly damage a very young fetus, doctors urge women who are trying to get pregnant to curtail their drinking.

A mother’s smoking can also lead to complications during delivery, particularly a condition called Meconium Aspiration Syndrome, in which an over-stressed baby passes fecal matter during labor and takes it into the lungs. That, in turn, can lead to life-threatening pneumonia.

Although the incidence of the syndrome among the sample was small, it was more than twice as likely to occur in babies whose mother’s smoked (0.35 percent) compared to mothers with no medical risks (0.16 percent).

Because the babies (of smoking mothers) have been less well-fed with blood and the food and the oxygen that comes with the blood (during pregnancy), they tend to be less able to handle the stress of labor and delivery.