Miscarriage Risk by Week: A Data-Driven Breakdown

Miscarriage risk starts at roughly 22–25% at week 4 and falls steeply through the first trimester, reaching about 3% by week 10 for women under 35 with a confirmed heartbeat. Each week of a healthy pregnancy is genuinely meaningful.

The question "what are my chances this week?" is one of the most searched things in early pregnancy — and for good reason. Most sources give you a single number like "1 in 5 pregnancies end in miscarriage," which tells you almost nothing about your odds right now. The miscarriage risk by week tells a more useful story.

This article breaks down the numbers week by week using figures from published obstetric cohort studies. All statistics are for women under 35 with no prior pregnancy losses, unless stated otherwise.

Why Risk Isn't Flat Across the Trimester

Most people think of the first trimester as uniformly risky, but the risk curve is steep and front-loaded. The majority of pregnancy losses happen in the first 6 weeks after conception (weeks 6–8 by gestational age). By week 10, around 75% of all first-trimester losses have already occurred.

The biology explains this. Early embryos are in rapid chromosomal division. Errors during this process — a missing chromosome, an extra one, a structural rearrangement — are far more likely to be lethal in the first weeks than later. Once an embryo clears these early milestones and cardiac activity begins, the underlying biology becomes more stable.

Week-by-Week Risk Estimates

Here are the approximate miscarriage risk figures for each gestational week for women under 35 with no prior losses. These draw from Cohain JS et al. (2017) and Agarwal K et al. (2011).

Week 4: ~25%

This is the week most people get a positive pregnancy test. Risk is highest here, partly because the figure includes "chemical pregnancies" — very early losses that resolve before symptoms appear. Many people experience a week-4 loss without ever knowing they were pregnant.

Week 5: ~20–22%

Risk remains high as the embryo implants and the placenta begins to form. hCG levels are rising rapidly but the pregnancy is not yet visible on ultrasound.

Week 6: ~12–14%

A meaningful drop. By week 6, a gestational sac and often a yolk sac are visible on transvaginal ultrasound. A fetal heartbeat may first be detectable this week, though it's not always visible yet.

Week 7: ~8–10%

Cardiac activity becomes reliably visible via transvaginal ultrasound. If a heartbeat is confirmed at week 7, your risk drops substantially — to around 3.5–4.5% for women under 35.

Week 8: ~5–7%

By week 8, embryonic development has passed several key milestones. The heart has four chambers. Brain structures are forming. Risk without a confirmed heartbeat is around 5–7%; with a confirmed heartbeat, closer to 2–3%.

Week 9: ~3.5–5%

The embryo is now technically a fetus. Most major organ systems are present, though immature. Risk continues to fall.

Week 10: ~2.5–4%

This is often when the first prenatal appointment happens. If you've reached week 10 with a viable heartbeat, your risk has dropped to around 2.5–3% without additional risk factors.

Week 12: ~1.5–2.5%

The end of the first trimester. Risk drops to under 2% for most women under 35 with a confirmed heartbeat. Many couples choose this week to share their news.

Weeks 14–16: ~1–1.5%

Second-trimester losses are rare and increasingly driven by structural or placental issues rather than chromosomal abnormalities. Risk is now well below 2%.

What Changes These Numbers

These figures are baselines. Three factors shift them meaningfully:

Maternal age raises risk at every week. A 40-year-old at week 8 faces roughly twice the risk of a 28-year-old at the same week. Our guide on maternal age and miscarriage risk covers this in detail.

Heartbeat confirmation cuts risk roughly in half at any given week. A confirmed heartbeat at week 8 is one of the strongest positive signs in early pregnancy. You can read more in our piece on miscarriage risk after heartbeat confirmation.

Prior pregnancy losses raise risk by 30–130% depending on how many prior losses you've had. One prior loss applies a 1.3× multiplier; three or more applies 2.3×.

You can combine all four factors — week, age, heartbeat, prior losses — using our miscarriage risk calculator to get a personalized estimate.

When the Numbers Plateau

By week 16, the week-by-week decline essentially levels off. From week 16 to term, loss rates are very low and driven by different mechanisms (placental insufficiency, cord accidents, structural abnormalities) than the chromosomal losses common in the first trimester.

This shift is why ACOG defines two clinical categories: "early pregnancy loss" (before 13 weeks) and "late pregnancy loss" (after 20 weeks). The biology, causes, and clinical management differ substantially between these periods.

How to Use This Information

These numbers can feel either reassuring or alarming depending on where you are in your pregnancy. A few grounding points:

• Risk at any individual week doesn't predict your outcome. These are population-level probabilities.
• Each additional week of a visible, healthy heartbeat is genuinely meaningful data. The descending curve is real.
• If you're monitoring a pregnancy closely after a prior loss, these figures give you a framework for understanding what each milestone means.

For a personal risk estimate combining all your factors, use our free miscarriage risk calculator. If you have ongoing concerns, your OB/GYN or midwife is the right resource for your specific situation.

Sources: Cohain JS et al. (2017). Spontaneous first trimester miscarriage rates. EJOG. Agarwal K et al. (2011). Miscarriage risk assessment. Br J Obstet Gynaecol. Nybo Andersen AM et al. (2000). Maternal age and fetal loss. BMJ.