DC 2007-04-27, In The Womb 子宫日记 12

At three months old, the fetus' most developed sense is hearing. The fetus may be completely surrounded by amniotic fluid, but because sound travels through fluid about four times faster than it does in air she has plenty to hear. The first sounds the fetus hears as her ears start picking up vibrations at 13 weeks are the gurgles and rumbles made by her mother's body. A succession of hiccups, burps, bubbles, sloshes and slurps marks the passage of food, liquid and air in, out and through the maze of passages and tubes just inches from the baby's ears.

The fetus also makes her own noises as she kicks and swishes in the amniotic fluid. She can also hear the competing flutter of heartbeats, her own racing at twice the speed of her mother's, both her constant companions during her time in the womb. The fetus can also hear sounds from the world outside: conversations, loud noises and music. The walls of the womb, together with the abdomen, act to filter out most of the high frequencies. All sounds reach the fetus distorted, but higher sounds are more muffled. Only the lower-bass notes of a piece of music have much impact.

Voice's sound is distorted too. Vowels are generally lower in pitch than consonants, so the fetus only hears the melody of speech without the percussion of consonants. The sound of the mother's voice is different from any other since it travels directly through the fluids of the body. This may help the baby develop the unique relationship it has with its mother. (Excellent, good luck, see you later, bye.)

Other voices like the father's must pass through air, then fluid and may not cut through the general background noise. (See, a little face there.) The loudest sound a fetus will ever hear may come during an ultrasound scan. It's impossible to hear the actual ultrasound waves. They are far too high at frequency to affect the human ear, but the ultrasound can cause secondary waves in the amniotic fluid that the baby can hear.

To produce finely-detailed images, ultrasound probes fire a rapid succession of pulses, each lasting less than one millionth of a second. The rapid switching on-and-off of these pulses can cause waves in the fluid which sound like a high-pitch tapping. If the probe is pointed directly at the baby's ear, it can sound as loud as a subway train.