Who it's for, and when
These positions are for pregnant women and their partners who want to go on enjoying each other, without forcing anything. They're simple and don't take much out of you.
As for timing, follow your belly rather than the calendar. Spooning is very handy throughout pregnancy, drawn-up thighs take over as the belly rounds out, and her on top particularly suits the final months. A nap, a moment of rest: the lying-down positions lend themselves nicely to both.
What to avoid as your belly takes up more room: missionary. He lies on top of you, and many women feel squashed and struggle to breathe. For the same reason, set aside positions where you lie on your front.
In the second half of pregnancy, also avoid lying flat on your back for long: the weight of the uterus then presses on a large vein (the vena cava), which can make you light-headed. If it happens, roll onto your side: the discomfort soon passes.
Health-wise, every pregnancy is different. At the slightest doubt or pain, or if anything worries you, stop and ask the doctor or midwife looking after your pregnancy for advice. The same goes for back pain or limited mobility: a health professional will point you in the right direction.
And these positions aren't just for pregnancy. Spooning with thighs drawn up works outside it too, and spooning remains a trusty ally on tired evenings.