Around the 12th week of pregnancy you will normally have a prenatal appointment where your doctor or midwife tries to hear the baby's heart beat. This is done with a Doppler, which is a device that uses ultrasound waves to detect a heartbeat when it is too early to detect it with a stethoscope.
Sometimes, at this appointment, you don't hear the baby's heart beat. This can be pretty frightening. There are many reasons why you don't hear the heart beat at this time, including:
Your Due Date is Wrong
If you weren't sure of your last period or if your due date is calculated using something other than ultrasound, your due date may be later than you thought, meaning you really aren't 12 weeks pregnant. This makes your baby's heart beat would be harder to hear.
If you have a tilted uterus, you may have a harder time hearing your baby's heart beat at first.
At this stage of pregnancy, your baby is very small. The Doppler needs to hit the baby in just the right way to catch the heart beat.
If you are overweight, sometimes the padding between baby and the Doppler is significant enough to cause it to be harder to hear the baby.
Sometimes the reason is because you are in the process of miscarrying, even if you have not had any signs of miscarriage.
If you have an appointment and do not hear the heart beat you can be told to come in a week or two to try again or you may be scheduled for an ultrasound to check on the baby.
Obstetrics: Normal and Problem Pregnancies. Gabbe, S, Niebyl, J, Simpson, JL. Fifth Edition.