A miscarriage, clinically defined as the spontaneous loss of a pregnancy before the 20th week, is a common event. Among women who have a confirmed pregnancy, the overall risk is estimated to be between 10% and 20%. Many miscarriages happen very early, sometimes before a person is even aware they are pregnant.
Miscarriage Probability by Gestational Week
The likelihood of a miscarriage is highest in the earliest weeks of gestation and decreases as the pregnancy progresses. In the initial weeks following conception, often before a pregnancy is even confirmed (weeks 3-4), the risk can be between 10% and 25%.
Once a pregnancy reaches week 5, the risk begins to decline. By week 6 of gestation, one study found the risk of loss to be approximately 9.4%. This figure drops to 4.2% at 7 weeks and further to 1.5% by 8 weeks. The detection of a fetal heartbeat on an ultrasound, which typically occurs between 6 and 7 weeks, is an indicator that the chance of a continuing pregnancy greatly improves.
Through weeks 9 to 12, the probability of miscarriage continues to fall, landing in the range of 0.5% to 5%. For the remainder of the period defined as miscarriage (weeks 13-20), the rate of pregnancy loss is estimated to be between 1% and 5%. Pregnancy loss after the 20-week mark is categorized as a stillbirth.
Why Miscarriage Risk Decreases Over Time
The drop in miscarriage risk as pregnancy advances is directly linked to embryonic development. The majority of early pregnancy losses, particularly those in the first trimester, are caused by chromosomal abnormalities in the embryo. These are random genetic errors that occur during fertilization or initial cell division, resulting in an incorrect number of chromosomes. An embryo with such an abnormality is often unable to develop properly.
The body’s natural processes can recognize when an embryo is not developing correctly, which often leads to a miscarriage. The successful progression of a pregnancy past these early weeks is a strong indication that the embryo has a normal set of chromosomes and is developing as expected.
Developmental milestones serve as evidence of a viable pregnancy. The formation and detection of a fetal heartbeat, for instance, confirms that the cardiovascular system is developing. As the fetus continues to grow and major organs form correctly, it demonstrates its own viability. Passing these developmental checks means the pregnancy has overcome the most common hurdles related to genetic viability, causing the statistical risk of loss to decline.
Factors Influencing Individual Miscarriage Risk
An individual’s specific risk is influenced by several factors. Maternal age is one of the most significant. The risk of miscarriage is lowest for individuals between the ages of 25 and 29, at about 10%. That risk increases with age, rising to about 25% for a 40-year-old and reaching 53% for someone aged 45 or older. This is largely because the quality of eggs declines with age, increasing the likelihood of chromosomal abnormalities.
Certain pre-existing maternal health conditions can also affect the risk of miscarriage if they are not well-managed. These include disorders like diabetes, thyroid disease, and autoimmune conditions such as lupus. Physical abnormalities of the uterus or a cervix that opens too early can also interfere with a pregnancy’s ability to continue. A history of previous miscarriages can also elevate the risk in a subsequent pregnancy.
Most miscarriages are not caused by a person’s daily activities or behaviors. The primary driver of early pregnancy loss is random chromosomal events that are beyond anyone’s control. Lifestyle factors like smoking, heavy alcohol use, and drug use are associated with an increased risk, but the vast majority of losses are due to factors related to embryonic development.