Survival of extremely premature babies in a geographically defined population: prospective cohort study of 1994-9 compared with 2000-5
BMJ 2008; 336 doi: https://doi.org/10.1136/bmj.39555.670718.BE (Published 29 May 2008) Cite this as: BMJ 2008;336:1221
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We read with great interest the article by David J. Field and coll.
(1) about survival of extremely premature babies in a geographically
defined population and the letter by Lisa Hilder (2) stressing among other
points the importance of assessing gestational age by early ultrasound.
We would like to underscore the importance of area based data
collection and we wish to report our experience from the Marche region
(centre Italy) where a prenatal network was started in 2003 with a
comprehensive data collection system including maternal, foetal and
neonatal variables. Data collection was performed for all pregnancies from
22 weeks and 0 days onwards by the local obstetrician; the local
neonatologists and verified by an external reviewer. In our region
gestational age was assessed by early ultrasound in more than 85% of
pregnancies. In 2003-2007 of the 69025 live births of the Marche region
109 occurred between 22-26 weeks, 70 (64%) were the infants alive at the
onset of labour and 56 (51%) were admitted alive to Neonatal Intensive
Care Unit. Stillbirth rate (death before labour) was 31%. Deaths in labour
were 6% (9% of infants alive at the onset of labor), and death in the
delivery room 7% (13% of live births). These figures appear to be lower
than those reported for the Trent region where death in labour was 18% and
death at delivery were 27% (1). It is thus of importance that these
figures should be given when evaluating regional survival rates. Infants
from 22 to 25 wks who survived to discharge were 48% with survival rates
of 0, 10, 78, 52% survival rates at 22, 23, 24 and 25 weeks respectively.
The EPICure study (3) report 34% and 52% survival rates (% of infants
admitted in Neonatal intensive Care Unit) at 24 and 25 weeks gestation.
The EPIpage study (4) report 64% and 79 % survival (% of infants admitted
in Neonatal intensive Care Unit) at 24 and 25 week repectively however
deaths in the delivery room was 38% (22-25 weeks). We underscore the
importance of area based data collection, the importance of having
gestational age assessed by early ultrasound, and we suggest that any
epidemiological studies involving infants at the limit of viability should
contain data on intrauterine death, deaths during delivery, and deaths in
delivery room.
1.Field DJ, Dorling JS, Manktelow BN, Draper ES. Survival of
extremely preterm babies in a geographically defined population:
prospective cohort study of 1994-9 compared with 2000-5. British Medical
Journal 2008; 336: 1221-1223
2. Hilder L., Improving survival is good, but preventing extremely
preterm birth is better. British Medical Journal 2008; Rapid responses18
May 2008
3. Costeloe K, Hennessy E, Gibson AT, Marlow N, Wilkinson AR. The
EPICure study: outcomes to discharge from hospital for infants born at the
threshold of viability. Pediatrics 2000;106:659-71
4. Larroque B, Breart G, Kaminski M, Dehan M, Andre M, Burguet A, et
al. Survival of very preterm infants: Epipage, a population based cohort
study. Arch Dis Child Fetal Neonatal Ed 2004;89:F139-44
Competing interests:
None declared
Competing interests: No competing interests
I read with interest the analysis of the trends in survival of babies
born very preterm to mothers normally resident in Trent (1). The study
found no change in survival of babies born at 22 and 23 weeks and better
survival when birth occurs at 24 and 25 weeks gestation. The rate of
survival to discharge of babies live born over five years from 2000-2005
at 22, 23,34 and 25 weeks gestational age in the period 2000-2005 can be
calculated as 0%, 11%, 18% and 49%. The one year survival rates for
babies born in England & Wales in 2005, the last year of the studied
by Field and his colleagues, were observed to be 5%, 16%, 42% and 65% for
babies born respectively at 22, 23, 24 and 25 weeks gestational age (2).
Although not comparable directly, these rates of survival over a longer
time from birth, for a much larger area, and obtained with different
methods have the same intriguing pattern of apparent improved survival
over time for babies born after 24 weeks, but little difference for babies
born at earlier gestations.
With such large differences in survival from one week to the next the
effect of gestational age misclassification is an important consideration.
In the national study concern about the accuracy of recording of
gestational ages was raised, especially as this affects indices at the
lower extreme of gestational age at birth (2). In the Trent study
measures were taken to reduce the possibility of transcription errors and
the method of gestational age ascertained was described. The validity of
nominating certain LMP dates as a more reliable method of assessing
gestational age than early ultrasound scanning is questionable(3), but
allows for a more consistent assessment of gestational age over the course
of t he study where the availability of dating ultrasound scans changed
from ‘perhaps 50%’ to ‘most’.
The starting point for the analysis undertaken by Field et al was
babies alive at the onset of labour. In many more pregnancies ending at
22-25 weeks fetal death occurred before this, or the timing of fetal
demise is not recorded. Fetal losses at this advanced stage of pregnancy
are as traumatic for women and their families as fetal or infant deaths
later in the pregnancy.
One important consequence of efforts to provide gestational age
statistics for England & Wales is the potential to contribute to high
quality surveillance and timely recognition of changes in survival at the
limits of viability. There are as many births in England & Wales
annually as in the twelve years needed for the analysis of trends in
extremely preterm babies in Trent. A particular challenge for such study
is the fact that no one system reliably captures all the relevant data
needed for surveillance and to monitor strategies aimed at preventing
these very preterm births.
References
1. Field DJ, Dorling JS, Manktelow BN, Draper ES. Survival of extremely
preterm babies in a geographically defined population: prospective cohort
study of 1994-9 compared with 2000-5. BMJ
http://www.bmj.com/cgi/content/full/bmj.39555.670718.BE. Accessed 9 May
2008.
2. Moser K, Macfarlane A, Chow YH, Hilder L, Dattani N. Introducing new
data on gestation-specific infant mortality among babies born in 2005 in
England and Wales. Health Statistics Quarterly 2007;35:13-27
www.statistics.gov.uk/downloads/theme_health/HSQ35.pdf
3. Bricker L, Garcia J, Henderson J, Mugford M, Neilson J, Roberts M,
Martin M-A. Ultrasound screening in pregnancy: a systematic review of the
clinical effectiveness, cost-effectiveness and women’s views. Health
Technology Assessment 2000; Vol. 4: No. 16
Competing interests:
None declared
Competing interests: No competing interests
Survival of extremely preterm babies, supersedes earlier response by Lisa Hilder, who has withdrawn it
Survival of extremely preterm babies
We read with interest the analysis of the trends in survival of
babies born extremely preterm to mothers normally resident in the Trent
Region.1 Although there are differences in detail, our analysis of
survival rates to the age of one year among all 645,887 babies born alive
in England and Wales in 2005 give a consistent picture, as the Table
shows.2,3 This compares, as far as possible, the data from the two
analyses, omitting deaths during labour from the Trent totals. The
differences between reported survival rates can be seen to be compatible
with random variation. As 2005 was the first year for which data about
gestational age have been available for England and Wales no data are
available for comparison with past years as was the case in the Trent
study. It is notable, however, that survival rates among babies born at 24
and 25 weeks of gestation are higher than those reported from UK studies
undertaken in the 1990s.4,5 As in the Trent study, survival rates were
much lower at earlier gestational ages.
With such wide differences in survival, the potential effect of
gestational age misclassification is an important consideration. In the
Trent Region study, measures were taken to reduce the possibility of
transcription errors. The strategy for ascertaining gestational age, was
consistent over the course of the study, although no detailed results were
provided.
In contrast, our data were derived from linkage of data derived from
routine clinical and civil registration practice. The gestational ages
were those reported by maternity units to the NHS Numbers for Babies
system.6 The accuracy of recorded gestational ages of 22 and 23 weeks in
our study is a particular concern. Checks carried out found internal
inconsistencies in the data for babies recorded as being born before 22
weeks of gestational age. Births where the gestational age was recorded as
less than 22 weeks and birthweights as 1,000g or over were therefore
excluded from our analyses. This was not done for births with gestational
ages of 22 weeks and above, however, so similar errors may therefore
remain. The effect of including these births is to increase apparent
survival and is likely to be more marked in the 22 and 23 week groups
where the numbers of live births were relatively small. In addition, some
of our registered deaths may have failed to link with births, and so our
survival rates could be slightly inflated.
Despite their very poor prospects, we would argue that larger
multiple ascertainment studies are needed before accepting that survival
is impossible for babies born at 22 weeks. We have no data to validate the
vital status or condition of the babies who appeared to have survived in
our study.
Many of the data in the Trent Region relate to small numbers of
births, even though data were aggregated for six year periods. Larger
population aggregates are needed to provide more timely recognition of
changes. The Office for National Statistics is now compiling these data
routinely to monitor trends in future years and this system has the
potential to contribute to high quality national surveillance of changes
in survival at the limits of viability. It will also include fetal deaths
at 24 or more weeks of gestation registrable as stillbirths, but there is
also a need for continuing voluntary monitoring of late fetal deaths at
earlier gestations. This is necessary to shift the emphasis of such
monitoring to include prevention of very preterm labour.
Table Comparison of survival rates of very preterm babies born alive
in the Trent Region and in England and Wales as a whole.
Alison Macfarlane
Lisa Hilder
Department of Midwifery,
City University London,
20 Bartholomew Close,
London EC1A 7QN
References
1. Field DJ, Dorling JS, Manktelow BN, Draper ES. Survival of
extremely preterm babies in a geographically defined population:
prospective cohort study of 1994-9 compared with 2000-5. BMJ
http://www.bmj.com/cgi/content/full/bmj.39555.670718.BE. Accessed 9 May
2008.
2. Moser K, Macfarlane A, Chow YH, Hilder L, Dattani N. Introducing new
data on gestation-specific infant mortality among babies born in 2005 in
England and Wales. Health Statistics Quarterly 2007;35:13-27
www.statistics.gov.uk/downloads/theme_health/HSQ35.pdf
3. Office for National Statistics. Preterm births, England and Wales,
2005, London: Office for National Statistics, 2007. Available online at:
www.statistics.gov.uk/StatBase/Product.asp?
vlnk=14882&Pos=&ColRank=1&Rank=272.
4. Saigal S, Doyle LW. An overview of mortality and sequelae of preterm
birth from infancy to adulthood. Lancet 2008; 371: 261-269.
5. Moser K, Macfarlane A, Dattani N. Survival rates in very preterm
babies in England and Wales. Letter. Lancet 2008; 371: 896-7.897.
6. Moser K, Hilder L. Assessing the quality of NHS numbers for babies
data and providing gestational age statististics. Health Statistics
Quarterly 2008; 37: 15-23.
Competing interests:
None declared
Competing interests: No competing interests