Epidemiology

It is estimated that some 50% of all conceptions abort spontaneously. About 35% of these are referred to as biochemical pregnancies. They are undetected by the mother - no prolongation of the menstrual cycle - but are indicated by the presence of beta-hCG 7 to 10 days after ovulation - denoting that conception has occurred.

Of the clinically recognised conceptions, the majority are lost in the first 14 days following conception. The rate of foetal loss from 8 weeks gestation to 28 weeks gestation is 3% on average.

Spontaneous miscarriage is influenced by:

• maternal age - foetal loss rate from 8-28 weeks is less than 2% in women under 30 years but rises to 5-10% in those over 40 years
• parity - increased risk of abortion with the higher orders of parity. This is independent of maternal age
• spacing of pregnancies - the optimal interval between pregnancies is 12-36 months. Other intervals have a slightly increased incidence of abortion
• previous fetal loss - a previous spontaneous abortion increases the relative risk of subsequent abortion
• previous prostaglandin termination - may result in uterine synechiae or cervical incompetence
• rate of spontaneous abortion is slightly increased in smokers; other toxic causes include alcohol and working with radiation

Earlier miscarriages are more likely caused by chromosomal abnormality including trisomy, polyploidy and autosomal monosomy. This accounts for:

• 60% of spontaneous abortions during 0-7 weeks
• 23% of spontaneous abortions during 8-12 weeks

Chromosomal abnormality remains a cause of spontaneous abortion in the 2nd and 3rd trimesters but it's importance relative to other factors diminishes.

An early miscarriage is the loss of pregnancy at <= 13 weeks' gestation (1,2)

• about one in four pregnancies, where a woman has missed a menstrual period and has a positive pregnancy test, ends in early miscarriage
• nearly 125 000 early miscarriages occur annually in the UK, accounting for 50 000 hospital admissions

Reference:

• Chu J et al. What is the best method for managing early miscarriage?BMJ 2020;368:l6438
• ACOG Practice Bulletin No. 200 Summary: Early Pregnancy Loss. (2018). Obstet Gynecol 2018;132:1311-310