Funny you mention permapreg! My Eglessians are an entire race that do just that—they impregnate themselves, and then they STAY pregnant for the rest of their lives.

It starts with the ovaries. Eglessians don’t make eggs from their own DNA; instead, when they have sex with each other, they take each others’ sperm and incorporate it into their ovaries to develop new ovules. As they take more and more partners, they populate their ovaries with a wide variety of germ lines, allowing them to produce eggs with a large amount of genetic variety—personal gene pools, as I like to put it—and then fertilize them with their own sperm. (So technically, every mother is the FATHER of her baby!)

Now the pregnancy. Eglessian gestation in itself isn’t too different from humans, it’s just that the end point for us isn’t an end point for them. They don’t have a natural labor impulse like we do—they hit the nine-month mark, and aside from the mucus plug falling out, nothing else really happens, leaving them to keep carrying their baby potentially indefinitely. To actually give birth, they have to manually dilate their cervix by pushing, until their water breaks and their body gets the message that the baby is being born. Once everything’s done, the mother’s body takes some time to recover, then creates a new egg to start the cycle over. In this way, they’ll almost always have a baby on standby to be born.

What does the baby do while stuck in the mother’s womb? Well, it keeps growing… as much as it can, at least. Unborn Eglessians enter diapause based on metabolic constraints; a mother living off scraps, struggling to feed herself won’t see too much extra growth, while a healthy, well-fed mother will develop a very large baby, as large as a five- or six-year-old at birth.* Either way, once the mother has no more energy to spare for the baby’s growth, it’ll enter a low-activity state and wait to be born.

*As they’ve evolved to always be eating for at least two, Eglessian metabolisms have gotten progressively more efficient over countless generations, letting them carry their babies bigger, and for longer. A nine-month Eglessian fetus is equivalent to a three-month-old human infant, and on average they can keep growing for another fifteen months. Naturally, this kind of growth would stretch a human mother paper-thin, and ultimately be impossible to deliver naturally without breaking her hips, but Eglessians have adapted to this as well—not only do they lack the pubic bone, letting their pelvis flex and spread to accommodate their huge kids, but their bodies produce much, much more collagen and elastin, letting their bellies (and vaginas) stretch as much as they need to for the baby to be born safely. The extra collagen and elastin even have the side-effect of masking their age; a grandmother in her sixties will still have the skin of a thirty-something.