Yes, we do know how this happens.
I'm due in the first half of October (the 12th, I think, if you're the sort who likes a date to fixate on).
We won't know if we're having a boy or a girl till some time early summer, but we probably will find out when the ultrasound is done.
We are not "trying for a boy" — we're hoping for healthy. All things being equal, we'd prefer a girl. We will, of course, love them regardless of sex or health.
I'm miserably sick, thanks for asking. As with the three previous pregnancies, I have "morning sickness" — that insane euphemism someone came up with for feeling nauseous round the clock and puking regularly if I'm not careful. I still maintain that if men had to experience this in order for the species to survive, we'd have cheap and effective treatments and/or the first trimester of pregnancy would be mandatory paid leave. </rant> I expect to feel better sometime around late April (if I'm lucky) or mid-May (if I'm not).
We've told the girls. Mairi is thrilled and touchingly solicitous of Mom's health. Nora seems happy but finds something new to worry about every few days. (She was already in this phase before being told about the impending sibling.) Dorothy is clueless.
If I missed your question please feel free to ask it and if I feel like it I'll answer in a later post...