The fall in serum hCG over 48 hours may be measured as the hCG ratio, which is calculated as:
  
    
      
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    {\displaystyle hCG~ratio={\frac {hCG~at~48h}{hCG~at~0h}}}
  
An hCG ratio of 0.87, that is, a decrease in hCG of 13% over 48 hours, has a sensitivity of 93% and specificity of 97% for predicting a failing pregnancy of unknown location (PUL).
Risk factors include: pelvic inflammatory disease, infertility, use of an intrauterine device (IUD), previous exposure to diethylstilbestrol (DES), tubal surgery, intrauterine surgery (e.g. D&C), smoking, previous ectopic pregnancy, endometriosis, and tubal ligation. A previous induced abortion does not appear to increase the risk. The IUD does not increase the risk of ectopic pregnancy, but with an IUD if pregnancy occurs it is more likely to be ectopic than intrauterine. The risk of ectopic pregnancy after chlamydia infection is low. The exact mechanism through which chlamydia increases the risk of ectopic pregnancy is uncertain, though some research suggests that the infection can affect the structure of fallopian tubes. Tube damage
Tubal pregnancy is when the egg is implanted in the fallopian tubes. Hair-like cilia located on the internal surface of the fallopian tubes carry the fertilized egg to the uterus. Fallopian cilia are sometimes seen in reduced numbers subsequent to an ectopic pregnancy, leading to a hypothesis that cilia damage in the fallopian tubes is likely to lead to an ectopic pregnancy. Women who smoke have a higher chance of an ectopic pregnancy in the fallopian tubes. Smoking leads to risk factors of damaging and destroying cilia. As cilia degenerate, the amount of time it takes for the fertilized egg to reach the uterus will increase. The fertilized egg, if it does not reach the uterus in time, will hatch from the non-adhesive zona pellucida and implant itself inside the fallopian tube, thus causing the ectopic pregnancy.