My own experiences:
I have been pregnant once; the father was my regular lover. He donated me blood on an almost nightly basis (I would gouge his back with my fingernails every time we had sex; he was efficient enough at producing endorphins that he never noticed any pain) and he displayed several vampiric traits himself, so i imagine the feeding was rather mutual. (Which might have explained my constant mood problems. If he was taking my life force more than I was taking his...)
I felt it "take" at the moment of conception. I felt the baby grow in me after that. It began to communicate with me in a very primitive fashion (very primitive - the brain hadn't exactly developed, after all) about four weeks after conception. I got a constant message of tenacious will to live. It knew that I was trying to kill it. I have very firm views on raising children in squalid poverty (I won't do it) and because my parents were very abusive, I'd never let any offspring of mine go to a foster home or adoption agency (my parents would have been accepted by such an agency with flying colours, you see. Some fates are worse than death. Had I know what family i was going to be born into, I would have preferred being aborted.)
I tried herbal cures which had always worked to bring on my menses. They grew increasingly toxic: I started with angelica root, which is fairly strong stuff, then tried a brew of pennyroyal, then tansy (which has been known to cause heart attacks). They didn't work. I knew my baby had inherited my poison tolerance. And my will to live and thrive no matter what.
My morning sickness was debilitating. I developed a nasty case of anemia, and the fact that I was already malnourished wasn't helping. I seduced my fencing instructor to make him think it was his baby, and got the money i needed for the surgical abortion that way, as the father of the child would never have been able to pay. I aborted my firstborn on Midsummer's day, though I hadn't planned the date. Obviously, the suction/aspiration method worked.
Anyway, I knew that I would have given birth to another vampire (which I certainly did not tell the doctor) and I had been told by a couple of medical opinions that I might not have survived the pregnancy because of the malnutrition; or it might have been my last baby. I think I would have survived. I can survive a hell of a lot. Still, it would have been a very rough pregnancy, I know that. I can't wait to try and breed again, when I have money. My body is healthier, so I suspect the pregnancy will be much better.
three years later, I donated eggs to an infertile woman (which seemed like a fair trade, karma-wise: to make up for the life I took out of this world, I'd help someone who couldn't get pregnant put a little life into the world). I shot up nightly with hormones to trigger hyper-ovulation. Most women on the drugs produce eight to fifteen eggs to be released at the end of the month. I produced forty.
I suppose that, too, is a form of reproductive efficiency.
I am pretty sure that the vampire gene (if this is a genetic, inborn condition - I'm seeing equal evidence both in my life and in readings for both the genetic theory and the viral theory) was passed down by my mother's side of the family. The Nalewajek/Studinski branch of my family tends to age slowly, and live long. My mother has nearly all of my health problems, and apparently there is something weird about her blood - she has more white blood cells than is normal. Her doctor mentioned that she appears almost to be allergic to herself, which I've seen mirrored in my own condition - all of my health problems (the nagging, annoying chronic ones) seem related to a hyperactive autoimmune system.
Twins do not run in my family. Longevity does. So do miscarriages, due to kidney problems, but technology these days is better than it used to be.
So does...a certain stubbornness. I don't know a single member of my family (either on the paternal or maternal side) who isn't pigheaded.