How does miscarriage risk actually change week by week?
The pattern of risk across the first trimester is one of the most striking declines in medicine. Understanding that pattern, in clinically accurate terms, can help make sense of what the statistics mean at different points in a pregnancy.
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How long does conception take?
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Miscarriage risk by week: what the data shows
Miscarriage occurs in approximately 10-20% of known pregnancies, according to the American College of Obstetricians and Gynecologists (ACOG). The true rate is likely higher because many losses happen before the pregnancy is confirmed — the Tommy's charity estimates that 1 in 4 pregnancies ends in loss when very early biochemical pregnancies are included. The 10-20% figure reflects the risk in clinically confirmed pregnancies and is the most commonly cited benchmark in medical practice.
The risk is heavily front-loaded in the first trimester and declines sharply as each week passes. At 4-5 weeks, before the embryo is visible on ultrasound, the chance of miscarriage is approximately 22-28%. By 6 weeks, it has fallen to approximately 10-17%. By 8 weeks with a confirmed heartbeat, it drops to approximately 3-5%. By 12 weeks (the end of the first trimester), the risk is below 2% for most age groups. By 14-20 weeks, the risk of pregnancy loss is approximately 0.5-1%. This steep week-by-week decline is the key insight most expectant parents search for: the risk does not stay constant — it falls dramatically with each passing week.
The data behind these estimates comes from multiple large-scale studies. Tong et al. (2008) tracked asymptomatic women with confirmed heartbeats at 7-10 weeks and found a subsequent miscarriage risk of approximately 2-3%. Macklon et al. (2002) documented that approximately 30% of all conceptions (including those before confirmed pregnancy) fail before clinical detection. The distinction matters: "1 in 4" and "1 in 8" are both cited correctly depending on which population is being measured. This calculator uses the week-by-week risk reduction data from Tong et al. and ACOG to show where risk currently stands for your gestational week.
When does miscarriage risk drop the most?
The sharpest decline in miscarriage risk occurs between weeks 6 and 9, particularly after a fetal heartbeat is confirmed on ultrasound. Before heartbeat detection (5-6 weeks), risk is approximately 10-15% in clinically confirmed pregnancies. Once a heartbeat is detected at 6-7 weeks, risk drops to approximately 5-10%. By 8-9 weeks with a sustained heartbeat, risk is approximately 2-3%. This means the heartbeat milestone is more meaningful as a risk predictor than gestational age alone — the same 8-week pregnancy with a confirmed heartbeat carries substantially lower risk than an 8-week pregnancy without one.
The end of the first trimester (week 12) is the traditional milestone for announcing a pregnancy because risk falls below approximately 2% at this point for most women under 35. By week 16, chromosomal abnormalities — the cause of approximately 50-60% of first-trimester losses — have almost entirely been weeded out by natural selection. Second-trimester losses (weeks 13-19) are significantly less common, affecting approximately 1-2% of pregnancies, and are more likely to involve structural issues, cervical factors, or infections rather than chromosomal causes.
How age affects miscarriage risk
Maternal age is the strongest individual predictor of miscarriage risk. For women under 30, the risk in a confirmed pregnancy is approximately 9-10%. For women aged 30-34, risk is approximately 11-12%. For women aged 35-39, risk rises to approximately 15-20%. For women aged 40-44, risk is approximately 25-35%. For women over 45, risk exceeds 50% in confirmed pregnancies. These are population-level figures; individual risk depends on additional factors including underlying health conditions and pregnancy history.
The age relationship is driven primarily by egg quality. As women age, a higher proportion of eggs have chromosomal abnormalities (aneuploidy) that prevent viable embryo development. At age 25, approximately 10-20% of eggs are aneuploid; by age 40, this rises to approximately 60-80%. Most chromosomally abnormal embryos result in miscarriage rather than live birth — a natural selection process that makes chromosomal miscarriage protective at the population level, though devastating at the individual level. The good news embedded in this data: miscarriage in women over 35 is more likely to reflect chromosomal issues specific to that conception rather than underlying fertility problems, meaning subsequent pregnancies often succeed. (Sources: ACOG Practice Bulletin 200, 2018; Tommy's, 2024; Tong et al., 2008.)
Frequently asked questions
The majority of first-trimester miscarriages (approximately 50-60%) are caused by chromosomal abnormalities in the embryo, most often a random error in chromosome number called aneuploidy. These chromosomal errors are not caused by anything the pregnant person did or did not do. Other causes include structural uterine abnormalities, hormonal problems, and immunological factors, but these are less common. In approximately 50% of cases, no definitive cause is found even after investigation. Lifestyle factors such as moderate physical activity, working, or normal daily activities do not cause miscarriage.
A miscarriage (also called spontaneous abortion in clinical terminology) is the loss of a pregnancy before 24 weeks of gestation in the UK (NICE definition), or before 20 weeks in the US (ACOG definition). Losses before 24 weeks are further classified as early (before 12 weeks) or late (12-24 weeks). Chemical pregnancies, which are losses that occur before a clinical pregnancy can be confirmed by ultrasound, are sometimes counted separately. The figures in this calculator refer to clinically recognised pregnancies.
In the UK, Tommy's charity provides free support from specialist midwives, a helpline (0800 0147 800, Monday to Friday), and online resources at tommys.org. The Miscarriage Association also offers support at miscarriageassociation.org.uk. In the US, the March of Dimes (marchofdimes.org, 1-888-663-4637) and the SHARE organisation (nationalshare.org) provide support for pregnancy and infant loss. Your GP or midwife can also refer you to specialist counselling services and, where appropriate, to a recurrent miscarriage clinic if you have experienced two or more losses.
NICE (UK) and ACOG (US) guidelines have moved away from recommending a waiting period after a first or second miscarriage. Research, including a large cohort study by Bhattacharya et al. (2010), found no evidence that waiting improved outcomes, and some studies suggest that conceiving within three months of a loss is associated with outcomes at least as good as waiting longer. However, this is a personal decision that should be made in consultation with a healthcare professional who knows your individual circumstances. Emotional readiness is as important as physical readiness and will vary significantly between individuals.
The figures in this calculator are based on large population studies but carry inherent uncertainty. Risk estimates vary across studies depending on how miscarriage was defined, whether pregnancies were identified biochemically or clinically, the population studied, and the time period. The figures are best understood as approximate population-level estimates rather than precise individual predictions. Additionally, they are averages across diverse populations, and individual risk will be affected by factors including age, previous pregnancy history, underlying health conditions, and other clinical variables that this calculator cannot fully capture.
A single miscarriage is not evidence of a medical problem. Approximately 50-60% of all first-trimester miscarriages are caused by chromosomal abnormalities in the embryo — a random event that is not caused by anything the parents did or did not do. ACOG and Tommy's both emphasise that one miscarriage does not indicate a pattern or underlying condition. Most people who experience a single miscarriage go on to have healthy pregnancies. Recurrent pregnancy loss — defined as two or more clinical miscarriages — occurs in approximately 1-2% of couples and warrants investigation to rule out uterine abnormalities, clotting disorders, chromosomal translocations, or hormonal issues. A single loss, while emotionally devastating, is a normal part of the reproductive experience at the population level. Seeking emotional support and allowing time to grieve is appropriate and important; seeking medical investigation after a single loss is generally not necessary unless there are other risk factors.
Miscarriage rates after IVF are higher than in natural conception for the same age group, primarily because IVF patients tend to be older on average. For IVF pregnancies using fresh embryos, miscarriage rates are approximately 15-25% for women under 35, rising to 40-50% for women over 40. Preimplantation Genetic Testing for Aneuploidy (PGT-A) — genetic screening of embryos before transfer — significantly reduces miscarriage risk in IVF by identifying and not transferring chromosomally abnormal embryos. For women using PGT-A-screened embryos, miscarriage rates typically fall to 5-10% regardless of maternal age. The use of frozen embryo transfers (FET) versus fresh transfers also affects outcomes: frozen transfers with PGT-A testing have become the standard of care at most IVF clinics because of improved implantation rates and reduced miscarriage rates compared to fresh transfers. (Sources: SART national IVF statistics 2022; NICE fertility guidelines 2023.)