Stillbirth Rate: Formula and Worked Example

The stillbirth rate is the number of fetal deaths divided by the number of live births plus fetal deaths, multiplied by 1,000. In the United States it is usually reported for fetal deaths at 20 or more weeks of gestation. This guide gives the formula, a worked calculation with the arithmetic shown, reading guidance, and snippets for Excel, R and Python.

What is the stillbirth rate? (definition)

The stillbirth rate is a vital-statistics measure of how often pregnancies end in fetal death late enough to count as a stillbirth. The National Center for Health Statistics (NCHS) defines the fetal mortality rate as the “number of fetal deaths per 1,000 live births and fetal deaths”, and notes that rates in the National Vital Statistics System are usually reported for fetal deaths at “20 weeks of gestation or more” (NCHS Data Brief 16, “The Challenge of Fetal Mortality”, checked 2026-10-29).

The National Institute of Child Health and Human Development (NICHD) states that in the United States stillbirth refers to the death of a fetus at or after the 20th week of pregnancy; a loss before 20 weeks is usually called a miscarriage or pregnancy loss (NICHD, “What is stillbirth?”, checked 2026-10-29). NICHD also splits stillbirth by timing: early (20 to 27 weeks), late (28 to 36 weeks) and term (37 weeks or more).

Because the numerator depends on the gestational-age cutoff, a rate is only comparable with another rate that uses the same cutoff.

Stillbirth rate formula (per 1,000 live births and fetal deaths)

Stillbirth rate = D / (L + D) × 1,000

D is the number of fetal deaths at or after the cutoff gestational age (20 weeks in the U.S. convention) in the place and period studied.
L is the number of live births in the same place and period.
L + D is the denominator: all births, live and fetal deaths. Fetal deaths stay in the denominator because they are part of the population at risk of the event.
1,000 is the scaling constant, so the result reads as “per 1,000”.

Stillbirth rate vs fetal mortality ratio vs perinatal mortality rate

The three names are often used loosely, so state the denominator every time. The NCHS definition above puts fetal deaths in the denominator. A ratio that divides fetal deaths by live births alone, D / L × 1,000, gives a slightly larger number from the same data. The difference is small when stillbirth is rare and grows as the rate grows. A perinatal mortality rate combines late fetal deaths with early infant deaths and has a different numerator and denominator; do not compare it with a stillbirth rate. Related measures on CASRAI include the standardized mortality ratio, the case fatality rate and the incidence rate ratio.

Worked example with the arithmetic shown

The numbers below are hypothetical, chosen so the arithmetic is easy to follow. They are not data for any real population.

A registry records 9,960 live births and 40 fetal deaths at 20 or more weeks in one year.
Denominator: L + D = 9,960 + 40 = 10,000.
Proportion: 40 / 10,000 = 0.004.
Rate: 0.004 × 1,000 = 4.0 stillbirths per 1,000 live births and fetal deaths.
For comparison, the ratio on live births only is 40 / 9,960 × 1,000 = 4.016, which rounds to 4.02. The two definitions differ by about 0.02 per 1,000 here, but a table that mixes them is still mislabelled.

Uncertainty with small counts

With 40 events the estimate is imprecise. An exact (Clopper-Pearson) 95% confidence interval for 40 out of 10,000 is 0.00286 to 0.00544, or 2.86 to 5.44 per 1,000. This is a standard statistical method applied to the hypothetical counts above; see the confidence interval entry for how to read it. Always report the interval when comparing small areas or short periods.

Interpretation: reading the result

A single rate has no meaning without a reference point. The table lists published values from CDC and NCHS sources for U.S. reference.

Reference point	Value	Basis
U.S. fetal mortality rate, 1990	7.49 per 1,000	Live births and fetal deaths, 20+ weeks (NCHS Data Brief 16)
U.S. fetal mortality rate, 2003	6.23 per 1,000	Same definition (NCHS Data Brief 16)
U.S. fetal mortality rate, 2005	6.22 per 1,000	Same definition (NCHS Data Brief 16)
Healthy People 2010 target	4.1 per 1,000	The 2005 rate exceeded it by 52% (NCHS Data Brief 16)
CDC current-burden statement	About 1 in 175 births, or about 5.7 per 1,000	CDC stillbirth data page; 1/175 × 1,000 = 5.71

Two readings follow from the table. Between 1990 and 2005 the rate fell from 7.49 to 6.22, a relative decline of (7.49 − 6.22) / 7.49 = 17%. NICHD describes the decline since CDC began collecting data in 1950 as having “leveled off in recent years”, with the early stillbirth rate down 3% in 2020 versus 2019 and the late rate relatively unchanged. The hypothetical 4.0 in the example would sit below the 6.22 of 2005 and close to the 4.1 target, but only a like-for-like comparison (same cutoff, same denominator, same period) supports that statement.

The CDC states that stillbirth affects about 1 in 175 births and that about 21,000 babies are stillborn in the United States each year (CDC, Stillbirth data and research, checked 2026-10-29). Convert such a “1 in N” figure to a rate with 1,000 / N.

Assumptions and common mistakes

Changing the cutoff. A 20-week rate and a rate with a later cutoff cover different events. Compare only like with like, and report the early, late and term split when the counts allow.
Wrong denominator. Dividing by live births alone, or by all pregnancies including losses before 20 weeks, changes the answer. State the denominator in the table header.
Small numbers. A move from 3 to 5 events in a small county is noise. Report the confidence interval, or pool several years.
Mixing “per 1,000” and “per 100,000”. Keep the scaling constant the same across a table.
Treating a rate as an individual risk. A population rate describes a group; it is not a prediction for one pregnancy.
Misreporting the event. A stillbirth is a fetal death. Do not describe the rate as the “infant mortality rate” or as a miscarriage rate. For general background on the discipline, see what is epidemiology.

How to report it (APA example)

Give the count, the denominator, the cutoff and the interval, and use APA 7th-edition numeric style (decimals for rates, brackets for the interval).

The stillbirth rate was 4.0 per 1,000 live births and fetal deaths (95% CI [2.9, 5.4]), based on 40 fetal deaths at 20 or more weeks of gestation among 10,000 births.

In a table, label the column “Stillbirths per 1,000 live births and fetal deaths” and note the gestational-age cutoff under the table. When citing the definition, name the source and URL, for example the NCHS Data Brief 16 address given above.

How to calculate it in Excel, R or Python

Excel

With fetal deaths in B2 and live births in C2:

=B2/(C2+B2)*1000

For the example, B2 = 40 and C2 = 9960 return 4.

R

d <- 40; l <- 9960
rate <- d / (l + d) * 1000          # 4
ci <- binom.test(d, l + d)$conf.int * 1000   # exact 95% CI per 1,000
round(c(rate, ci), 2)

Python

from scipy.stats import beta
d, l = 40, 9960
n = d + l
rate = d / n * 1000                               # 4.0
lo = beta.ppf(0.025, d, n - d + 1) * 1000         # 2.86
hi = beta.ppf(0.975, d + 1, n - d) * 1000         # 5.44
print(round(rate, 2), round(lo, 2), round(hi, 2))

The Python interval is the exact Clopper-Pearson interval, the same one R’s binom.test returns.

FAQ

What is the stillbirth rate?

It is the number of fetal deaths per 1,000 live births and fetal deaths, usually counted at 20 or more weeks of gestation in the United States.

How do you calculate the stillbirth rate?

Divide fetal deaths by the sum of live births and fetal deaths, then multiply by 1,000. With 40 fetal deaths and 9,960 live births, the rate is 40 / 10,000 × 1,000 = 4.0.

How common is stillbirth in the United States?

The CDC states that stillbirth affects about 1 in 175 births, roughly 21,000 babies each year.

What gestational age counts as a stillbirth?

In the United States, the death of a fetus at or after the 20th week of pregnancy (NICHD). Earlier losses are usually called miscarriage or pregnancy loss.

Is the stillbirth rate the same as the fetal mortality rate?

In NCHS usage the fetal mortality rate is the number of fetal deaths per 1,000 live births and fetal deaths, which is the stillbirth rate defined here. Confirm the cutoff and denominator before comparing any two published figures.