The first meaning is 'metre' which is a measurement of length. If the 'm' stands alone it is metre. The second meaning is 'milliâ¦' which means a thousandth. In this case 'm' can not stand alone, we need to know what we have a thousandth of; if it is weight, length or some other thing. Example: mg means 'milligram', a thousandth of a gram. Example: mm means 'millimetre', a thousanth of a meter.
At about six weeks into pregnancy (four weeks from conception), you should be able to see your baby's heartbeat on ultrasound. The typical heart rate at this age is about 100 beats per minute.
If an early ultrasound is not done, the first chance to hear a baby's heartbeat is at about 10 weeks of gestation using a fetal Doppler, which is a handheld ultrasound device. Baby Center points out, however, that it is more common to pick up a heartbeat with a fetal Doppler device at 12 weeks.