Of those who seek treatment for bleeding during pregnancy, about half will miscarry. Miscarriage may be detected during an ultrasound exam, or through serial human chorionic gonadotropin (HCG) testing. Risk factors
Miscarriage may occur for many reasons, not all of which can be identified. Risk factors are those things that increase the likelihood of having a miscarriage but do not necessarily cause a miscarriage. Up to 70 conditions, infections, medical procedures, lifestyle factors, occupational exposures, chemical exposure, and shift work are associated with increased risk for miscarriage. Some of these risks include endocrine, genetic, uterine, or hormonal abnormalities, reproductive tract infections, and tissue rejection caused by an autoimmune disorder. Trimesters
First trimester
Most clinically apparent miscarriages (two-thirds to three-quarters in various studies) occur during the first trimester. About 30% to 40% of all fertilized eggs miscarry, often before the pregnancy is known. The embryo typically dies before the pregnancy is expelled; bleeding into the decidua basalis and tissue necrosis causes uterine contractions to expel the pregnancy. Early miscarriages can be due to a developmental abnormality of the placenta or other embryonic tissues. In some instances an embryo does not form but other tissues do. This has been called a "blighted ovum".Successful implantation of the zygote into the uterus is most likely eight to ten days after fertilization.
Therefore, there may be a need for follow-up pregnancy tests to ensure that there is no remaining pregnancy, including ectopic pregnancy. An incomplete miscarriage occurs when some products of conception have been passed, but some remains inside the uterus. However, an increased distance between the uterine walls on transvaginal ultrasonography may also simply be an increased endometrial thickness and/or a polyp. The use of a Doppler ultrasound may be better in confirming the presence of significant retained products of conception in the uterine cavity. In cases of uncertainty, ectopic pregnancy must be excluded using techniques like serial beta-hCG measurements. A missed miscarriage is when the embryo or fetus has died, but a miscarriage has not yet occurred. It is also referred to as delayed miscarriage, silent miscarriage, or missed abortion.A septic miscarriage occurs when the tissue from a missed or incomplete miscarriage becomes infected, which carries the risk of spreading infection (septicaemia) and can be fatal.Recurrent miscarriage ("recurrent pregnancy loss" (RPL) or "habitual abortion") is the occurrence of multiple consecutive miscarriages; the exact number used to diagnose recurrent miscarriage varies. If the proportion of pregnancies ending in miscarriage is 15% and assuming that miscarriages are independent events, then the probability of two consecutive miscarriages is 2.25% and the probability of three consecutive miscarriages is 0.34%. The occurrence of recurrent pregnancy loss is 1%.