Most miscarriages happen before 12 weeks of pregnancy, with the highest risk concentrated in the earliest weeks, often before many people even know they’re pregnant. About 80% of all pregnancy losses occur within the first trimester, and the risk drops sharply as each week passes.
The Highest-Risk Weeks
The single riskiest period is the first few weeks after conception, roughly weeks 4 through 6 of pregnancy (counting from the first day of your last period). Many of these very early losses are called chemical pregnancies, where a fertilized egg implants briefly enough to produce a positive pregnancy test but stops developing before anything is visible on ultrasound. Chemical pregnancies account for an estimated 50 to 75 percent of all miscarriages. Many occur so early that the only sign is a late, heavy period, and they often go unrecognized entirely.
Once a pregnancy is visible on ultrasound and a heartbeat is detected, the odds shift significantly in favor of the pregnancy continuing. Among women who reach six weeks, the chance of the pregnancy continuing is about 78%. A visible heartbeat at eight weeks raises that figure to 98%, and by ten weeks it climbs to 99.4%. These numbers come from research on women with a history of recurrent miscarriage, so for someone without that history, the outlook at each milestone is likely even more favorable.
Why Early Losses Are So Common
Chromosomal abnormalities cause roughly half of all first-trimester miscarriages. These are random errors that happen when the egg and sperm combine, resulting in an embryo with too many or too few chromosomes. The embryo simply can’t develop normally, and the pregnancy ends on its own. This isn’t caused by anything either parent did or didn’t do. It reflects the biological reality that human reproduction has a high rate of genetic error at the very start.
Because chromosomal problems tend to prevent development from the outset, they cluster heavily in the earliest weeks. As pregnancies progress past the point where these errors would cause a loss, the remaining risk drops considerably.
How Risk Changes Week by Week
There’s no single chart that gives an exact percentage for every gestational week, but the overall pattern is well established. Risk is highest in weeks 4 through 6, declines noticeably once a heartbeat is confirmed around weeks 6 to 8, and drops dramatically after week 12. By the end of the first trimester, the chance of miscarriage in an ongoing pregnancy with a confirmed heartbeat is very small.
Clinically recognized miscarriage, meaning a loss confirmed by a doctor after a positive test or ultrasound, occurs in about 10% of all pregnancies. When you include the very early chemical pregnancies that happen before most people seek medical care, the true rate is higher, but those losses are biologically distinct from a miscarriage at, say, 9 or 10 weeks.
After 12 Weeks
Losses after 12 weeks are called late miscarriages, and they are significantly less common. The causes also tend to be different. Rather than chromosomal errors, late losses are more often linked to structural problems with the uterus or cervix, infections, blood clotting disorders, or issues with the placenta. After 20 weeks, a pregnancy loss is classified as a stillbirth rather than a miscarriage.
What Affects Your Individual Risk
While the week-by-week pattern applies broadly, certain factors shift the baseline risk up or down. Age is the most significant variable. Miscarriage rates rise with maternal age, largely because chromosomal errors in eggs become more frequent over time. At age 25, the overall risk of miscarriage is around 10%. By 40, it’s closer to 40 to 50%.
Other factors that increase risk include a history of two or more previous miscarriages, smoking, heavy alcohol use, and certain chronic conditions like uncontrolled diabetes or thyroid disorders. On the other hand, a pregnancy that shows a strong heartbeat and normal growth on early ultrasounds carries a lower risk regardless of age, because the most common cause of loss (chromosomal abnormality) would typically have ended the pregnancy before that point.
If you’re tracking your pregnancy week by week and feeling anxious, the key milestone to keep in mind is the heartbeat confirmation, usually at your first ultrasound between 7 and 9 weeks. Once you pass that point, the statistical picture changes substantially, and each additional week that passes makes a loss less and less likely.