Then I think back to the 60's - when women smoked while they were pregnant.. (Heck - right in the hospital..) No low-birth weight babies; no babies with health issues; and the mother's are still alive as well.. Why?
But here's the thing. Our understanding of "babies with health issues" has changed dramatically since the 1960s. I graduated nursing school in the 1970s and went straight to work in a labor & delivery dept in a big hospital. At that time doctors and nurses still smoked on the units, even right at the nurses station. Cigarettes were sold in the gift shop and in the vending machines. During their labor, women were allowed to smoke to calm their nerves. I can remember one particular New Years that i worked some of the nurses brought in wine and we toasted the new year with the doctors AND the patients.

While on duty!
Back then, any baby born before 36 weeks was considered premature, just like today. However, back then they didn't have the rescue equipment and medications to save near-term or preterm infants so a lot of times decisions were made to withhold any treatment to make the end faster. I know.

There were no TV style rescusitation scenes. If the baby was strong enough to breathe, it did. If not, it died and was labelled a miscarriage. Very rarely would a 34 week premie make it.
My MIL had her last baby at 37 weeks. She was a pack a day smoker and drank a cocktail every day. The baby was born normal weight, but had what was termed hyaline membrane disease, what we would call respiratory distress syndrome today. Today they would have popped him onto a vent for a bit and he would have almost certainly have pulled through in a few days. It is very unusual today for a baby that near term to die from premature lungs. Back then (1961) losing a baby to such a "miscarriage" was shockingly famliar. But even then, doctors didn't make the connection between smoking and the infants' lungs. It was thought that since the baby didn't inhale smoke that the lungs would be protected. Back then they believed that the placenta filtered out any "bad things" the mother might ingest or inhale. Of course, we know better now.
So, I'm sure that for every person who smoked and drank that popped out a series of perfect, normal weight babies there are a host of others whose babies were "miscarried" due to the lack of oxygen in the womb. They just didn't know what was causing it until later.
