Now, a new industry has popped up to satisfy their craving for an audible reminder: fetal heart monitors for use at home. Hand-held devices that are similar to the ones in doctors' offices, which use Doppler ultrasound to amplify the heartbeat, are widely available on the Web at sites like http://www.babybeat.com and http://www.heartones.com. They can be rented for about $20 to $50 a month, or purchased outright for between $150 and $600. Several thousand monitors are rented or sold each year to families, according to varying estimates from manufacturers.

It's all part of a move to cater to expectant parents who want to exert the same control over childbirth that they do over their careers and lifestyles. Fixating on every step of pregnancy, some are moving to monitor every aspect of their child's development - even before birth. The Doppler monitors are just the latest pricey, high-tech offering in a list that includes three-dimensional ultrasound, umbilical cord-blood storage, and early mail-away blood tests that can tell you the sex of your unborn baby.

The problem with the heart monitors, however, is that many Web sites are skirting the law by renting and selling them. Fetal-heart monitors are in a class of medical devices regulated by the Food and Drug Administration that patients must have a prescription to purchase.

The concern is that people using the devices without any supervision might overuse them and harm their unborn children by exposing fetuses to more ultrasound than is necessary.

In recent months, two sites, http://www.heartbeats athome.com and http://www. fetalsure.com, changed their policies and began requiring prescriptions following a call from a reporter asking how they handle the FDA requirements.

Other sites, however, merely post a note on their Web site suggesting that users discuss the matter with their doctors; some, like http: //www.storkradio.com, ask a buyer to pledge that they've consulted a licensed medical professional about the use of the device.

The easy availability of the Doppler monitors has gotten the attention of regulators. The penalty for procuring a medical device without a prescription can be as high as $16,500 per violation. But Julie Zawisza, an FDA spokeswoman, says the agency is highly unlikely to prosecute a parent, since the FDA focuses on entities that it regulates, including device manufacturers and retailers. The FDA plans a public meeting later this year to address the question of whether the monitors should be available without a prescription.

Ultrasound technology has been used in obstetrics for nearly 50 years, and dozens of studies have turned up no ill effects on the fetus during normal usage. The devices that are being used at home typically involve a small wand that is held over the belly and is attached to a base unit with speakers that emit the signature, rhythmic sound of a fetal heartbeat. But none of the studies have involved extended, unsupervised home use by expectant parents. ''Simply because no adverse effects have been revealed, it is not acceptable to conclude that no adverse effects exist,'' wrote Linda Kahan, an FDA deputy director, in response to one physician's petition to allow over-the-counter sales of the monitors.

Ultrasound sends sound waves into the body and uses the echoes bouncing back to create images. Most ultrasound uses short bursts of sound to generate images. Because the bursts are so short, the risks are believed to be minimal during normal use in a medical setting.

Continuous Doppler ultrasound, which is what most of the hand-held fetal monitors use to let people listen to the fetus's heartbeat, deploys acoustical energy that is lower in intensity than standard ultrasound. But the energy needs to be emitted more continuously for the machines to do their work. The potential risk here is that it could heat and damage developing fetal tissue if used for long periods of time.

Despite this theoretical risk, most doctors use these machines themselves. That's because the risk of not using the machines - and missing an emerging developmental problem - is much greater than the chances of any potential tissue-heating causing harm.