Why did (do) states used to require couples to get a blood test before getting a marriage licence? 
Was it to test for VD?
Yes, for syphilis. With the major aim being to prevent congenital syphilis in offspring
However, it was never really cost-effective. And when the frequency of syphilis declined, it became less so. It cost a state tens of thousands of dollars (and even more in some states) to identify a single previously unidentified case of syphilis via pre-marital blood testing.
Sensibly, most states finally gave up the practice.
Here’s a nice summary. http://sti.bmjjournals.com/cgi/reprint/77/3/214.pdf
Nvm, the Doc has answered.
Isn’t there also an issue with the RH factor? I seem to recall something about the possibility of complications during pregnancy, if the mother’s type differs from the fetus’s, which it would be good to be forewarned about.
There’s also things like Thalassemia where if both have the minor form, there is the chance that any children of theirs could have the major form of it. Which from reading up on it, I wouldn’t want a child of mine to have it (I have the minor).
But the RH factor is tested as part of the blood tests taken when you get pregnant.
In a couple of the states that still test nowadays, they test for rubella immunity in the female.
Rubella infection during pregnancy can have extreme consequences for the fetus, and a vaccination can be given before pregnancy, if needed.
Rh factor can be an issue in pregnancy (if the mother is Rh- and the fetus is Rh+), but there has been a medication around since, I believe, about 1968 called Rhogam that can prevent the problems from occuring.
I thought that grew hair on your legs!
Political medicine is almost always ineffective and usually expensive.
There is a test for a congenital condition (PKU) that can detect the absence of an essential enzyme for metabolizing milk. Knowing the condition exist, one can prevent horrifying consequences of the absence with very simple treatment. So, it makes sense to test every child for the presence of the enzyme. State legislatures all over the United States jumped right in.
So, now, every child is tested at birth for the presence of the enzyme. Great! Except that the enzyme doesn’t get into the blood until the child has had milk in their diet. Brest fed babies don’t get any milk the first day or so. But, you can’t be sure that you will ever see the baby again, and the law says you (the doctor) must do the test. So, they stick a pin in the kid’s foot for purely legal reasons, right after birth. Then, most good physicians tell mom that they need to repeat the test in a few days.
But the first test must be done, even though it is widely known to be unreliable.
Tris
Well, actually, in New York at least it cost the state nothing. The two times I married, the test was done by our doctor and we paid for it.
Right, if there were no treatment, there’d be little point in knowing whether you had an RH mismatch. But if you do know in advance, then you know to pursue the treatment.
But most doctors will treat all Rh- mothers as if their unborn child is Rh+. You just never know - or believe - a woman that the father of her baby is her husband. You know what I’m saying? Just bad for the malpractice insurance rates if you get that one wrong, since it is so easily preventable.
I’m Rh-, and Rhogam treated even though I had absolutely zero question as to who my baby daddy is - my also Rh- husband - or the date of the conception. Didn’t matter. Just in case God Hisself is Rh+ and arranged a miraculous* conception the moment before my husband’s swimmers got there.
*(notice I didn’t say immaculate, aren’t y’all happy?)
On reflection, I guess in some infinitesimally small percentage, there is a chance that the blastocyst could have had some genetic mutation and an Rh- potential could turn into Rh+. Had to have happened at least once, for the first Rh+ person ever, right?
Hm, I suppose there’s some logic in that. I was just considering that there must be some side effects to the treatment, so if the father was known (or at least, alleged) to be negative, it might be better to pass on it. But I guess the side effects must be relatively minor.
That’s a good point. I’ve done this twice (which means three shots - WhyBaby was born without one of hers since everyone was in such a hurry to get her out 
), and no one ever mentioned any side effects. It was just not presented as “optional” treatment. Since I know how important it is and that there’s no other option other than nothing (and my son, the first born, is indeed Rh+, from another relationship - so he may indeed have poisoned the well, so to speak), I didn’t press.
Looking it up now, it does seem as those adverse effects are extremely rare, and generally limited to redness and swelling at the injection site which responds to antihistamines or corticosteriods, or simply the Rhogam not working, and antibodies being formed anyway. The adverse effect rate is approximately one complaint per 60,000 doses distributed for use, with no deaths.
That’s a pretty mild and low rate of side effects, even for this pharmaceutical skeptic.