Early Pregnancy Monitoring
Human chorionic gonadotropin (HCG) is a hormone specifically produced by a pregnancy, and the detection and measurement of hCG form the basis of all pregnancy tests. The presence of hCG can be detected within a day or two of the time of expected menses, and hCG levels are used to monitor the early progression of pregnancies. As a general rule, the hCG level will approximately double every 48 hours in normal early pregnancies. If this rate of rise is not present, closer observation and testing may be warranted until the cause of the low levels of hCG is determined.
Vaginal ultrasound can demonstrate the presence of an intrauterine pregnancy as early as 10 days after a missed period. This corresponds to an hCG level of approximately 2,000 mIU/ml to 3,000 mIU/ml. Based on the initial hCG level, and taking into account the 48-hour doubling time, many practitioners schedule the first ultrasound when the hCG level will be approximately 5,000. An ultrasound at this time answers several questions: Is the pregnancy in the uterus? Does it look normal? How many are there?
If the pregnancy is in the uterus, a gestational sac can be seen. This is a fluid-filled sac that on ultrasound looks like a dark hole. The gestational sac should contain a fetal pole, which is composed of the early fetal tissue. Seeing a fetal pole tells us with a certain level of reliability that the pregnancy is probably normal. The presence of a fetal pole does not ensure 100 percent that the pregnancy is normal, but a relatively small percentage of pregnancies in which a fetal pole has been demonstrated will miscarry. Finally, we can see how many gestational sacs are in the uterus.
Early Obstetrical Ultrasound Showing a Fetal Pole
If the ultrasound fails to demonstrate a pregnancy within the uterus, an ectopic pregnancy must be suspected. If the hCG levels do not rise appropriately, the same is true. If the ultrasound demonstrates a gestational sac but no fetal pole, this is known as an empty sac, or blighted ovum. This is not a normal pregnancy (the vast majority of these have abnormal chromosomes) and is destined to miscarry.
If an ultrasound performed two to three weeks after the first ultrasound, or four to five weeks after the missed period, demonstrates that the fetus has continued to develop and a heartbeat is present, the likelihood is very high that the pregnancy will be successful.