At six weeks, some doctors perform a sonogram, also called an ultrasound, to determine the health or viability of the developing embryo. This is generally the earliest that anything is visible on an ultrasound, although at this point it will not look much like a baby, since development is still in the early stages.
At this early in a pregnancy, it is difficult to see anything at all using a standard external ultrasound machine. Often, if a six-week sonogram is performed at all, it will be using a vaginal ultrasound machine, which consists of a wand-shaped device that is inserted into the vagina.
A six-week ultrasound can determine the location of the embryo and ascertain that it is in the correct place in the uterus. If the pregnancy is an ectopic pregnancy, with the embryo implanted outside the uterus in the Fallopian tube, this can be determined based on the blood flow patterns seen via ultrasound.
The fetal heartbeat is often detectable at a six-week ultrasound. The normal fetal heartbeat at six weeks is about 90 to 110 beats per minute. Detecting a heartbeat at this stage indicates that the pregnancy will probably continue and not end in miscarriage, although this is not absolutely guaranteed. If the sonographer cannot detect a heartbeat, the pregnant woman will generally be advised to come back in another week or two to check again, since sometimes a heartbeat that is undetectable at six weeks may be stronger and more noticeable at seven weeks or more.
The fetal pole is the basic overall shape of the embryo, which can be seen via sonogram at around the six-week mark. The fetal pole resembles a bean in shape, which the technician can look at and determine the head and rump ends of the embryo. Seeing the fetal pole allows the sonographer to measure the size of the embryo.
Chorionic Sac and Yolk Sac
The chorionic sac, sometimes called the gestational sac, is the circular sac of liquid that encases the fetus throughout all of its development in the womb. The yolk sac sits within the chorionic sac and provides nourishment to the embryo until the placenta has been established and nutrients begin to flow in through the umbilical cord. Both the chorionic sac and yolk sac should be visible in a six-week ultrasound.
Since many women do not have perfectly regular menstrual periods, dating of a baby in the womb can sometimes be off by a few days, or even a few weeks. Ultrasounds performed at early points in the pregnancy, such as a six-week sonogram, may be inaccurate due to mistakes in dating how old the pregnancy actually is. If the dates are off by a week and the embryo is actually only five weeks old, for example, things that should be visible, such as the fetal pole or heartbeat, may not be detectable yet. Because of this uncertainty in dating, doctors will usually advise a repeat sonogram in a week or so if anything unusual is detected.