4. How frequently do early miscarriages normally occur?
In most cases, miscarriages are quite a shock for the woman or couple concerned. After the first joy upon hearing the good news, there is frequently worry at the onset of heavier bleeding, and then the bitter disappointment when the foetus passes, or when the physician finds indications of a malformation in the foetus that will lead to miscarriage. The couple will then anxiously ask whether the miscarriage was an event that can simply happen, or whether they have a basic defect that will or can always lead to miscarriages. There are, in fact, disorders which increase the probability of miscarriage, and these need to be established or excluded. Our question here concerns the frequency with which miscarriages occur with completely healthy couples. They are astonishingly frequent and have no bearing on future pregnancies. This fact is certainly reassuring for couples who have experienced such a miscarriage. Wilcox et al. have made a study of several hundred healthy women who wanted to have children. They collecting the
women's urine daily over several cycles (a total of 707 cycles) and determined the time of ovulation and the increase and further course of the pregnancy hormone, human chorionic gonadotropin (HCG), by measuring the hormone levels in the daily urine samples. They furthermore recorded the appearance, duration and severity of menstruations. 198 pregnancies occurred, of which a total of 59 (= 31 %) ended as miscarriages. In 43 cases the miscarriages occurred before the pregnancies had been clinically diagnosed by sonogram (earliest miscarriage). In many cases the women did not even know they had been pregnant. The only thing they had noticed was that menstruation had set in a little later and was also more severe. The only indication of a brief pregnancy were the temporarily positive HCG levels in the urine, which was established later. In 9 % of the women, the miscarriage took place after the pregnancy had already been clinically established. 95 % of the women with a very early miscarriage became pregnant again within the following two years, suffering no further complications. A large number of these miscarriages is certainly due to the fact that approximately 50 % of all ovae have a genetic defect. These results were obtained from women whose ovaries were stimulated with medication. However, much information points to the assumption that this also holds true for the ovae in a natural cycle. There are many obstacles to be overcome between fertilization and an intact pregnancy. From in-vitro fertilization we know that a large number of fertilized ovae do not develop into embryos which can successfully implant in the mucous tissue of the uterus. This state clearly increases with the age of the women. In such cases the ovum dies before implanting in the uterus. In some cases it just manages to settle, but does so later than usual. A delay in the rise of the pregnancy hormone levels in the blood and the urine after ovulation is an indication of delayed implantation or delayed embryonic development already prior to implantation, and the greater the delay, the greater the likelihood that the pregnancy will end as a miscarriage.
What this means in practice: It is nothing out of the ordinary to have one or two miscarriages. More frequent miscarriages, however, can hide a problem that should be recognized at an early stage.
Reference List
WrambsbyH, Fredga K, Liedholm P (1987) Chromosome analysis of human oocytes recovered from preovulatory follicles in stimulated cycles, N Engl J Med 316:121-124
Wilcox AJ (1988) Incidence of early loss of pregnancy. 319: 189-194
Wilcox A J, Baird D D, Weinberg C R (1999) Time of implantation of the conceptus and loss of pregnancy. N Engl J Med 340: 1796