Most countries' medical associations do not recommend ultrasounds unless there is a clear medical indication for one. But in practice, routine ultrasounds are very common. Some pregnant women even have one ultrasound during each prenatal appointment, while others only have one. It is unlikely that you will be offered an ultrasound before 12 weeks of pregnancy. Before this time, abdominal ultrasounds are not always effective at detecting a fetus and a heartbeat. Therefore, if you do have a medical reason for an ultrasound before this time, like having a miscarriage symptom, ultrasounds would be carried out trans-vaginally.
If you are only going to have one ultrasound during pregnancy, 20 weeks gestation is perhaps the ideal timing for it. During this time, the placental location that will be accurate by the time your baby is born is reasonably likely to determine. You will be able to hear your baby's heartbeat easily, as well as detect any obvious birth defects and the gender of your baby too. Later on in pregnancy the position of your baby can be determined by palpating the uterus. Most skilled midwives and some OBs will be able to do so very accurately. Ultrasounds do carry some risks, which is why some avoid them and why medical associations support the opinion they should be carried out when there is a medical indication.