I can see it when I believe it ‐ or at least define it

Abstract

I thought I knew what the word meant… I do know its meaning when I see it. I can even speak about it or know when not to. It is at the heart of much of the research in the field of reproduction, and usually it means “all systems go”. Indeed, we'd be hard pressed to find an adult who didn't know about it; even kids in preschool have insight into the phenomenon. Yet when asked to find a biological definition that works across the animal kingdom, most are stumped - I certainly am. Pregnancy, that is. Pregnancy is when an embryo/fetus/offspring is developed within the body of its parent, resulting in a live birth (Blackburn, 2000). Some dictionaries make it even easier to grasp: “the state of having offspring developing in the body”, or “the developmental period from conception to birth within a female”. Pregnancy is a specifically evolved reproductive strategy that humans use - and this is all about humans, of course. Okay, and maybe monkeys, and even some other primates… oh yeah, and most mammals. But, if we then consider the vast diversity of reproduction, we quickly realize that its basic definition could very well apply to organisms that we wouldn't otherwise suspect. Angiosperms, for instance, house developing embryos within seeds, which are in turn contained within fruit, and the fruit remains attached to (continuous with) the adult… at least until it is ripened. Such a scenario clears the dictionary's high-bar definition of pregnancy - having offspring develop within the body - but it seems inappropriate to refer to a fruiting peach tree as “pregnant”; we can at least exclude the pregnant-peach problem on grounds that the majority of development takes place only after the pit has fallen to … well … the ground. (Safe on a technicality!) We still run into problems even among animals. Chickens, for instance, fertilize internally, but we do not consider gravid hens pregnant since they lay and brood eggs outside the body. Still, if we closely consider the situation at hand, there is a short period in which the embryo develops within the reproductive tract. Must we then concede that hens are pregnant between conception and egg laying? The young in the eggshell is certainly alive. Okay, let's raise the definition bar and add something about nutritional and homeostatic benefits of mammalian-style pregnancy. That should take care of it, and will perhaps exclude those difficult animals that utilize specialized brooding mechanisms (e.g. mouth, back, and side pouches), as do many frogs, some sea anemones, and even sea stars - after all, these adults do not offer much in the way of nutrition, just protection. We can also add a restriction that the developing organism is not only within the body, receiving nutrients, but also continuous with it. Argh! Then female hammerhead sharks, for instance, would be capable of pregnancy. They maintain a placental link to their pups before delivering them live. Although this animal does not have bones, it does have a placenta! Actually, it usually has a bunch of placentae! Hammerhead shark eggs are large and full of yolk - this is where the developing embryo gets its nutrition, at first. When the yolk sac is depleted, though, the developing animal stays within the female reproductive tract and the empty yolk sac transforms into a structure analogous to the mammalian placenta - often called a “yolk sac placenta”. This organ acts as a conduit between mother and pup, providing sustenance to the offspring until its birth. Sound familiar? With a litter of 12 to 15 pups (20–40 in the , Great Hammerhead) (Compagno, 1984), that is a lot of placentae. But is she pregnant? … And for that matter, are kangaroos ever pregnant? They have a uterus, the fetus does have a placenta (as undeveloped as it is), and the fetus does receive nutrition. But the “fetus” is born so prematurely - months before it is capable of leaving the pouch! This is not easy…. Taking stock of these considerations, a rigorous definition of pregnancy might be: “The condition whereby an embryo develops within and is continuous with its parent, receiving nutritional and homeostatic benefits before live birth.” This definition is stringent enough to work across the animal kingdom, but remains flexible enough to accommodate the variation we expect in the field of reproduction. Yay! Now I can see it, because I believe it! And of course this is all occurring within a female… only… Right? Ryan Walsh Jessica Laird Gary Wessel