Prospective risk of unexplained stillbirth in singleton pregnancies at term: population based analysis

Unexpected late fetal death is tragic but not uncommon, most such fetal deaths being unexplained. Although five times more common than sudden infant death,¹ they have attracted scant public attention.

Delivery is recommended when the risks to the fetus in utero are greater than those to the baby after birth; in high risk pregnancies this is generally believed to be around 38 weeks. The risk of unexplained stillbirth near term is, however, relevant to all pregnancies. Current numerical estimates do not detail risk by gestation,¹ and the few studies that have done so are no longer applicable in the United Kingdom in the late 1990s. Yudkin et al calculated the total risk of stillbirth by gestation using population data that are currently over 15 years old,² while Feldman calculated a prospective risk using data from a New York City population, including multiple pregnancies and a high proportion of women with no antenatal care.³ We calculated prospective risks of unexplained stillbirth by gestation in singleton pregnancies near term.

Methods and results

We reviewed published data on 171 527 births in the North East Thames region in 1989-91⁴ and derived the number of ongoing pregnancies and stillbirths at or beyond each gestational week from 35 to 43 weeks. The prospective stillbirth rate per 1000 ongoing pregnancies was calculated as the number of stillbirths at or beyond week n divided by the number of pregnancies at or beyond week n multiplied by 1000, where n is the week of gestation from 35 to 43 weeks.

As the original dataset included all stillbirths (explained and unexplained and those in multiple pregnancies), we applied correction factors to derive gestation specific risks of unexplained stillbirth in singleton pregnancies near term as follows. We used data from the Office for National Statistics for 1994 to estimate the proportion of all births (live births and stillbirths) that were singleton (650 826/659 545=0.9868) and the proportion of overall stillbirths that were in singleton pregnancies (3465/3813=0.9087). We used data from the 1994 confidential enquiry into stillbirths and deaths in infancy¹ to estimate the proportion of total stillbirths of fetuses >2500 g that were unexplained (833/1137=0.7326).

The table shows the risk of stillbirth in ongoing pregnancies. At or beyond 38 weeks one in 730 singleton pregnancies were complicated by an unexplained stillbirth at term and one in 529 by stillbirth of any cause. Stillbirths of any cause may be more relevant because all stillbirths beyond 38 weeks are arguably unexpected since fetuses with recognised risk factors have usually been delivered by this time.

Comment

We acknowledge that the risks we report are approximations, being derived from three sources of data, but they provide the first quantifiable estimate of risk in continuing singleton pregnancies near term. This information is relevant to modern obstetric practice, where women want to be informed and have high expectations about the safety of their unborn child. Most women would want a caesarean section if the risk of fetal death or damage to their child exceeded one in 4000.⁵ Our calculations show that the risk of stillbirth at term is five to eight times higher than this.

Interestingly, at 38 weeks the risks of stillbirth near term exceed those at 42 weeks, when delivery is usually recommended. Delivering women routinely at 38 weeks would lead to a high incidence of caesarean section with its attendant risks, either primarily or from failed induction, in addition to a small risk of iatrogenic neonatal respiratory morbidity.

Antepartum stillbirth is a major public health problem, accounting for a greater contribution to perinatal mortality than either deaths as a consequence of prematurity or the sudden infant death syndrome.¹ Research into the underlying mechanisms and aetiological factors of this problem to identify pregnancies at risk must remain a prerequisite for any selective strategy to prevent these deaths.

Table.

Unexplained stillbirths and total prospective risk of stillbirth by gestation

Gestation (weeks)	Total No of ongoing pregnancies	No of singleton ongoing pregnancies	No of stillbirths per week	Total No of stillbirths at or beyond each week	No of singleton stillbirths at or beyond each week	Total No of stillbirths /1000 ongoing pregnancies	Total No of singleton stillbirths/1000 ongoing singleton pregnancies (95% CI)	Prospective risk of total stillbirths in singleton pregnancies	No of unexplained singleton stillbirths/1000 ongoing pregnancies (95% CI)	Prospective risk of unexplained stillbirth antepartum in singleton pregnancies
35	164 860	162 684	54	489	444	2.97	2.73 (2.48 to 2.99)	1:366	2.00 (1.79 to 2.23)	1:500
36	162 603	160 457	73	435	395	2.68	2.46 (2.22 to 2.72)	1:407	1.80 (1.60 to 2.02)	1:556
37	158 171	156 083	56	362	329	2.29	2.11 (1.89 to 2.35)	1:474	1.55 (1.36 to 1.76)	1:645
38	149 181	147 212	83	306	278	2.05	1.89 (1.67 to 2.12)	1:529	1.37 (1.20 to 1.58)	1:730
39	127 160	125 481	72	223	203	1.75	1.62 (1.40 to 1.86)	1:617	1.19 (1.00 to 1.39)	1:840
40	93 828	92 589	81	151	137	1.61	1.47 (1.24 to 1.75)	1:680	1.08 (0.88 to 1.31)	1:926
41	39 316	38 797	50	70	64	1.78	1.65 (1.27 to 2.11)	1:606	1.21 (0.89 to 1.61)	1:826
42	10 328	10 192	16	20	18	1.94	1.77 (1.05 to 2.79)	1:565	1.30 (0.68 to 2.18)	1:769
43	1 883	1 858	4	4	4	2.12	2.15 (0.59 to 5.50)	1:465	1.58 (0.33 to 4.71)	1:633