Whether loud noise is harmful to the developing embryo and fetus, and whether exposure to loud noise warrants removal of pregnant women from noisy environments remains unclear. This paper addresses the potential of loud noise on the developing human fetus. Four general areas of the topic are reviewed: acoustic physics, noise attenuation in utero, the medical literature about noise exposure, and considerations for pregnant women in the work force.
Sound is defined as the sensation produced by stimulation of the organs of hearing by vibrations transmitted through air. The apparent loudness attributed to sound varies not only with its pressure on the eardrum, but also with its frequency or pitch. Frequency may be defined as vibrations per second or Hertz (Hz). Decibels are another way to characterize sound measurement. When the decibel is used to measure sound, a reference point is implicit. This reference point is usually considered to be 0 dB, the starting point on a sound scale. The starting point is the weakest sound that can be perceived by an adult in an extremely quiet location. Table 1 lists sources of loudness by decibels in daily living.
The maternal body seems to attenuate the transmission of external sound to the fetus. This attenuation ranges from 20 dB at low frequencies, to 70 dB or more at high frequencies. Background noise in utero is about 85 dB, with peaks of 95 dB reported with each stroke of the mother's heart. Seemingly, the fetus can tolerate the 85 to 95 dB of background noise experienced in utero.
One area of concern is that noise in pregnancy may be stress producing. Sudden noises greater than 80 dB elicit a startle reflex in nonprepared adults. Paradoxically, a willing listener does not experience this startle reflex on exposure to sounds of up to 120 dB. Little is known about the fetal response to loud noises.
Other authors have concluded that fetal hearing may be impaired by noise louder than 90 dB. These reports have not taken into account the following: 1) the background noise level of 85 to 95 dB experienced in utero; 2) the noise attenuation of the maternal body. No reports with adequate controls for noise exposure have demonstrated in utero hearing loss because of external noise.
There are several reports about aircraft noise and its effect on pregnancy. Consistent findings have been decreases in birth weight for women living in close proximity to large airports. Unfortunately, these reports fail to take into account confounding variables, including socioeconomic status, and tobacco use. No report has measured noise levels in the home of the subjects.
Because of the paucity of information on the effects of prenatal noise exposure, our knowledge is incomplete. It remains difficult to recommend any definitive medical guidelines for prenatal noise exposure beyond the protection of maternal hearing. The pregnant woman should not unnecessarily be removed from the work force and employers should protect all of their workers from excessive noise.