Lies, Damned Lies, and Miscarriage Statistics

Trying to figure out your chances of miscarrying? Sadly, you are going to have a hard time finding good information. 

Many websites claim to tell you your risk of miscarriage, citing statistics that look like these:

But problems abound with their numbers.

Problem 1: These sites rarely provide their sources, so you cannot tell whether their information is reliable.

Problem 2: These sites do not breakdown miscarriage risk by other known risk factors, like the mother’s age.

Problem 3: Nearly all these sites derive their statistics from just two small studies, one which tracked 222 women from conception through just the first 6 weeks of pregnancy, and another which tracked 697 pregnancies, but only after a fetal heartbeat had been detected–a key point, because heartbeat detection dramatically lowers the chances of a miscarriage.

The lack of good information frustrated me when I was pregnant, and I bet it frustrates you too. So I have compiled a summary of the best research on risk of miscarriage. Where possible, I break down the risk by…

• pregnancy week
• before a heartbeat has been detected 
• after a heartbeat has been detected 
• fetal heart rate
• mother’s age
• father’s age
• number of prior miscarriages

Edit: I also have a new post on how morning sickness signals a lower risk.

Risk of Miscarriage by Pregnancy Week

Miscarriage risk drops as pregnancy progresses. The risk is highest early in the first trimester. Fortunately, for most women by 14 weeks their chance of a miscarriage is less than 1%.

Miscarriages rates declined between 6 to 10 weeks, according to a study of 697 pregnancies with a confirmed fetal heartbeat:

• 9.4% at 6 weeks
• 4.6% at 7 weeks
• 1.5% at 8 weeks
• 0.5% at 9 weeks
• 0.7% at 10 weeks

A similar study of 668 pregnancies with a confirmed fetal heartbeat between 6 and 10 weeks, found a similar decline in miscarriage risk by week:

• 10.3% at 6 weeks
• 7.9% at 7 weeks
• 7.4% at 8 weeks
• 3.1% at 9 weeks

But for women in their mid to late 30s and early 40s, these studies understate the risk. Even after confirmation of a fetal heartbeat, miscarriage risk remains high for women 40 and older through 12 weeks, according to a study of 384 women 35 and older. 

Despite the higher risk for this age group overall, a normal ultrasound result from 7 weeks remains a promising sign. Women who entered the study in their 4th to 5th week of pregnancy had about a 35% risk of miscarriage. Women who entered the study later, and who therefore had a normal ultrasound and heartbeat at 7-10 weeks, had a risk under 10%.

Miscarriage Risk by Fetal Heart Rate

A fetal heartbeat often indicates a healthy, viable pregnancy. But a fetal heart rate that is too slow can instead signal an impending miscarriage.

The chance of a first trimester miscarriage varies by fetal heart rate, according to a study of 809 pregnancies. The lower the heart rate, the higher the miscarriage risk. (Normal fetal heart rates change with fetal age, so these tables break down the risk by pregnancy week.)

Up to 6 weeks 2 days gestation:

Between 6 weeks 3 days and 7 weeks 0 days:

After 7 weeks, the fetal heart rate was at or above 120 beats per minute for almost all ongoing pregnancies.

Miscarriage Risk by Week Before Confirmation of a Heartbeat

Many women will not have an ultrasound and fetal heartbeat confirmation until sometime between 8-10 weeks. What are their chances of a miscarriage before that crucial piece of news?

In a large prospective study of 4,887 women trying to conceive, 4070 became pregnant. Their rate of miscarriage was 4-5% in week 6. By week 7, this risk fell to 2.5%. Rates hovered around 2% per week until week 13, when chances of a miscarriage dipped below 1%

Personal Risk Factors

Your personal characteristics and behaviors alter your miscarriage risk. The most important risk factor, as is well known, is the woman’s age: Miscarriage rates climb as women age, especially after the late 30s. The man’s age matters too, especially after they turn 40.

Risk of Miscarriage by the Woman’s Age

Anne-Marie Nybo Anderson, of the Danish Epidemiology Science Centre led the largest population-based study ever conducted on age and miscarriage. Anderson tracked every “reproductive outcome”– every pregnancy, miscarriage, birth, stillbirth, or abortion–in Denmark between the years of 1978 and 1992–ultimately tracking outcomes of over a million pregnancies.

What did she find? Miscarriage risk rises sharply during a woman’s late 30s and reached nearly 100% by age 45.

Rates of ectopic pregnancy also rose with age:

As did the chances of a stillbirth:

(In Anderson’s study, stillbirth was defined as a loss after 28 weeks. In the U.S., any loss after 20 weeks is usually considered a stillbirth)

Take heart though: as scary as the rise in stillbirths sounds, the risk remains under 1% through age 45.

Anderson’s study’s findings parallel those of another large and well-studied sample: U.S. pregnancies conceived via IVF.

Just as in Anderson’s study of Danish pregnancies, the uptick in miscarriage risk among IVF pregnancies begins at age 38.

Intriguingly, the overall miscarriage rates among IVF pregnancies is lower than in the Denmark sample. This is probably due to selection effects. Only some women manage to become pregnant through IVF, and embryos transferred during IVF are chosen based on early signs of normal development. Passing through these early hurdles likely ups the odds of a successful pregnancy.

Risk of Miscarriage by the Man’s Age

Researchers often ignore the man’s age when studying miscarriage. Most women marry men who are about the same age, so researchers have trouble teasing apart the effects of the woman’s age from the man’s age. 

Fortunately, several studies have now included couples in which either the woman or the man is much older than their partner.

These studies provide a clear and consistent picture: older prospective fathers raise the risk of miscarriage by about 25-50%. One study found an a 60% increase in the odds of a miscarriage if the father was over 40. Another reported a roughly 25% increase in the risk of miscarriage for fathers over the age of 35.

Other studies report similar effects; all showing most marked rise after age 40 (see here and here).

Risk by the Couple’s Combined Age 

A young partner can offset some of your personal age-based miscarriage risk, especially if you are a man. Men whose partners are young, under 30, have relatively low chances of miscarriage regardless of their own age, according to large retrospective European study.

For women, alas, young partner only partially offset their age-based risk. Women over 35 with relatively young partners, under age 40, still face double to triple the odds of women in their 20s.

Older partners do, however, compound the risk for women in their 30s. A woman in her early 30s with a partner over 40 has roughly triple the odds of a woman with a partner the same age or younger. 

Risk of Miscarriage After Confirmation of a Fetal Heartbeat for Older Women

On a more positive note, women in their late 30s and early 40s have a good chance of an ongoing pregnancy after confirmation of fetal heartbeat.

For women over 40, once a heartbeat has been detected at 7-10 weeks, the risk of a miscarriage falls to around 10%. After 20 weeks, the risk plummets to less than 1%.

How Does a Prior Miscarriage Affect Your Risk of Miscarriage?

Aside from age, the best predictor of whether a woman will miscarry is the number miscarriages she has already suffered. Most websites quote these statistics:

From these statistics, one prior miscarriage seems inconsequential; while just two prior miscarriages appears to dramatically raise your chances of another miscarriage.

Fortunately, these statistics are too dire for women who have had two prior miscarriages. The outcomes from a study over a million pregnancies paints a much more reassuring picture, at least for women who have had fewer than 3 prior miscarriages

Here’s the risk of a subsequent miscarriage for women who have never given birth before:

And for women who have given birth before:

The Bottom Line

In early pregnancy, miscarriage risk falls with each passing week, with significant drops around the 7-week mark, and again after the 12-week mark.

Your age, your partner’s age, and your number of prior miscarriages all affect your overall risk of miscarriage. Miscarriage risk rises dramatically after about age 37 for women, and age 40 for men.

References

Ammon Avalos, L., Galindo, C. and Li, D.-K. (2012), A systematic review to calculate background miscarriage rates using life table analysis. Birth Defects Research Part A: Clinical and Molecular Teratology, 94: 417–423. doi: 10.1002/bdra.23014