Animals in utero: An inside view

Dolphins, for example, develop tiny leg-like limb buds during their first month in the womb, but the buds vanish within two weeks, and the animals swim in amniotic fluid for the next 11 months of gestation. Scientists say that little trick of evolution is a sign that the highly intelligent mammals descended from dog-like land creatures.

Speaking of one of history's most successful mammals, dogs' characteristic panting begins in the fetus just weeks before birth. That behavior helps the dog regulate its own body temperature in the absence of ample sweat glands.

And the elephant, Earth's largest terrestrial mammal, begins to develop unusual lungs at month 4 of a 22-month pregnancy that will allow the animal to breathe through its trunk when crossing rivers as an adult. Scientists think that could link the mammal to an evolutionary past as an aquatic animal.

It may be hard to picture, but that's just what National Geographic has tried to do for its upcoming television feature In the Womb: Animals, which is set to air Sunday at 9 p.m. PST on the National Geographic Channel. The two-hour program charts the fetal development of three mammals--the dolphin, dog and elephant--with special imagery rarely seen outside a zoologist's office.

Scientists used advanced ultrasound technology, called 4D (for four dimensions), to capture three-dimensional images of the animals inside the womb. (The ultrasound transmits high-frequency sound waves to a region of the body, which then return echoes to produce pictures of the fetus.) Unlike traditional ultrasound imagery, 4D images depict the length, width and depth of the fetus in multiple shots over a period of time.

"It's like taking a 2D sonogram and making it into a motion picture," said Jenny Apostol, executive producer at the National Geographic Channel. "That's what enables us to create a real window into the behavior of animals in utero."

Video: Never-before-seen images of critters in the womb
Upcoming program watches animals before birth.

National Geographic also hired special effects artists to create models of the developing mammals that would augment the 4D imagery. The artists built models of the mammals in clay, then in silicon, a process similar to production for a computer animation film. The artists then use the models to craft digital versions of the mammals in a continuous motion picture throughout gestation.

To get an ultrasound of the dog, a veterinarian cuts back some of its fur and applies a water-soluble gel, just like with an ultrasound of a human. The vet then moves a transducer over the dog's stomach in circular fashion while it's still awake. The process could confirm a pregnancy within 12 days after conception, but it takes between 25 to 30 days to detect the number of pups.

It's a little trickier to get an ultrasound of an elephant. The vet must first perform an enema to rid the animal of all its feces and clear a path in the rectum for the ultrasound transducer to be inserted. (The elephant is held in place with ropes.) The vet then inserts the probe to get images of internal organs. But because of the elephant's size, the process only yields partial views on the ultrasound screen. Vets can confirm a pregnancy within 16 weeks of conception.

A dolphin ultrasound is likely the easiest to pull off. A dolphin can be trained to float on its side before the procedure. Once ready, the vet will run the transducer over the dolphin's body for the probe. The technology detects a dolphin pregnancy about four months into gestation.

According to producers of the film, ultrasound technology has not shown any harm to human or animal fetuses after 50 years of tests.