By the ninth or tenth week of pregnancy the fetal heart can be heard with an ultrasound detector. Sound waves encounter tissues and bounce back in an echo phenomenon, and moving tissues will change the pitch of the sound. This is known as the Doppler effect. Ultrasound waves at two million cycles per second – the frequency that is used – cannot be heard by the unaided human ear, but changes in the pitch of their echoes from a moving object can be converted electronically and made audible. This means that the beating heart of the fetus can be heard.
If you are heavy, or the fetus is in a particular position, it may be difficult to pick up the heart tones at this early stage of pregnancy. Generally, if your uterus is growing normally and you have not experienced any problems such as cramping or bleeding, this is no cause for concern. Some physicians and midwives do not even attempt to locate the fetal heart tones this early in pregnancy, waiting instead until around 16 weeks to hear with a Doppler or even until 20 weeks to hear with an ordinary stethoscope or specially designed fetoscope at about 20 weeks.
At about 20 weeks of pregnancy, the movement of the fetus can be felt, not only by the mother, but also by an outside observer using a hand on the mother’s abdomen. The mother’s perception of movement is called quickening and is considered a presumptive symptom of pregnancy. By the time and outside observer can feel fetal movement it can be considered a definite sign.
Today, ultrasound performed through the vagina (transvaginal ultrasound) can detect a gestational sac - the earliest positive evidence of pregnancy – in the uterus at about 10 days after implantation. This is just about the time of the first missed menstrual period. Using ultrasound performed with the instrument placed on the abdomen, a pregnancy may be confirmed by 4 to 5 weeks after your last menstrual period – about 2 to 3 weeks after fertilization. These are definitive symptoms of pregnancy. [an error occurred while processing this directive]