The reason this is important is that women have an increased risk of tubular (ectopic) pregnancy if their tubes are tied and tubal reversal actually slightly increases the risk. Most doctors that do tubal reversals report 10-30% ectopic pregnancies, our rate is 2.5%. I believe that doing the tube in layers and dye testing the tube to make sure it is open, which is why our ectopic rate is so much lower.
Recent research has been published showing that the rate of increase of the HCG value is higher in patients where the initial value is lower. That rate also increases less when the initial value is higher. Our patients are always warned of the risk of ectopic pregnancy and told to get a blood pregnancy test as soon as their urine test is positive. Once the blood HCG is above 2000 an ultrasound needs to be done to confirm the pregnancy is in the womb and not in the tube. If the ultrasound does not show a uterine pregnancy it is okay to repeat both tests in 2 days as long as there are no symptoms of an ectopic pregnancy (most commonly light-headedness and severe pain). The problem is that many woman have pain early on in a normal uterine pregnancy and they may also get dizzy.
The rise in the HCG value can help identify a pregnancy that is in the uterus and not in the tube. You don’t want to operate or medically treat for a suspected ectopic and ruin an intrauterine pregnancy. Research data suggests if the first value is 1500 or less it should increase by 49% in two days. If the first value is 1500-3000 it should increase by 40%. If it is greater than 3000 it should increase by 33%. This is a new way to look at the HCG test. The diagnosis of an ectopic pregnancy is usually one of exclusion. You often don’t see the ectopic pregnancy in the tube on an ultrasound but will also not see it in the uterus. If a woman’s HCG level falls in the appropriate range the chances of everything being okay increase and its okay to wait and repeat the tests as indicated.
If you are a physician reading this I would refer you to Obstetrics and Gynecology Volume 128, Number 3, September 2016 pp 504-511.