Expanding on the critically acclaimed special, In the Womb, National Geographic explores the extraordinary and fragile world of twins, triplets and quadruplets in utero. Featuring amazing 4-D ultrasound images and revolutionary new fetal imaging techniques, this remarkable new special travels inside the womb to witness tiny fetuses as they grow and begin to interact with each other.

This documentary follows the multiple pregnancies of three different women, from conception to shortly after the birth. Rachel (the only one who isn’t a first-time mother) is having fraternal twins (a boy and a girl), Jennifer is having triplets (two identical boys plus one fraternal brother who was conceived on a different day), and Julie is having identical quads (girls). All three of these sets of multiples were conceived naturally, even Julie’s (the odds of having naturally-conceived identical quads are one in eight million!).

Using cutting-edge 4-D technology, we’re able to get a fascinating look at just what goes on in the womb during these gestational periods. Not only do we get to see what the embryos/fetuses look like as they develop, but we also get to see the kinds of interactions they have with one another, interactions that vary from up-close and personal (if they’re together in the same amnion and chorion, no barriers between them) to only able to feel one another through membranes if they’re in separate amnions and chorions.

This documentary is loaded with just about everything you ever might have wanted to know about multiple pregnancies. Topics include the different types of twinning, what might cause conjoined twins, how interactions in utero continue playing out after birth (such as one set of twins in which the dominant twin often kicked his brother, who put his head down on the placenta for comfort instead of fighting back.