Between the fifth and sixth months (approximately 24 weeks), the heartbeat is strong enough to be heard with a special instrument called a foetoscope.    From Wordnik.com.  [Chapter 6] Reference

The size of the woman's womb is measured to check the baby's growth and, after 20 weeks of pregnancy, the baby's heart may be listened to with a special stethoscope called a foetoscope.    From Wordnik.com.  [Chapter 9] Reference