Reviewer's report
Title:Reference population for international comparisons and time trend
surveillance of preterm delivery proportions in three countries
Version: 1Date:17 March 2008
Reviewer: emile papiernik
Reviewer's report:
1. The question posed by the authors is well defined. Their aim is to use
what they call a reference population to monitor trends in preterm birth
rates in three Scandinavian countries.
2. The methods do not seem to be appropriate to the aim. The authors define
a sub population that represents only one third of the total population,
without giving enough justification. For instance, one would expect that
their criteria would be homogenous. However, primiparity is not homogenous.
Women with no previous pregnancy can have lower preterm birth rates than
women with no previous pregnancy, but with a previous induced abortion, or a
late second trimester abortions as shown by Ancel (references below).
The epidemiological knowledge on preterm birth shows the importance of risk
factors that are essential for understanding the evolution of the rate of
preterm delivery. For instance, twin pregnancies increased after more
widespread use of ovulation induction drugs. If attention is focused on
primarily on the reference population, which excludes women with induced
ovulations and twin and higher order pregnancies, what knowledge does that
provide about the reality of risk factors and the evolution of preterm
births? The same question applies to the issue of the evolution of the risk
of preterm birth as more women have children later in life.
The limits of their proposal for using a reference population are revealed
by its inability to shed light on the increase of preterm births in Denmark.
By focusing on the idea of a reference population, the authors neglect to
analyze the trends present in their data. For instance, the authors do not
discuss the fact the increase in preterm birth is apparent not for severe
preterm births 24-27 w, but for preterm births at higher gestation duration
. The third surprising result which is not discussed by the authors is why
the preterm birth in the whole population of Denmark did not increase as
much (5.3% to 6.1% in the whole population but less than 4.4% to 6.8% in the
reference population).
3. The data are sound for the hypothesis defended by the authors
4. The manuscript is well presented
5. The discussion section is not adapted, if one does not accept the
hypothesis of the authors. They need to present a better defence of their
thesis.
6. The limitations of this work have not been presented
7 . I.do not know of other papers on this topic.
8. The title reflects their aim. They present in the summary that they have
observed an increase in preterm the rates in Denmark, however,they do not
explain why they have no explanation for this phenomenon
9.The writing cannot be appreciated by a French speaking reviewer.
Ancel PY, Lelong N, Papiernik E, Saurel-Cubizolles MJ, Kaminski M; EUROPOP.
History of induced abortion as a risk factor for preterm birth in European
countries: results of the EUROPOP survey.
Hum Reprod. 2004 Mar;19(3):734-40. Epub 2004 Jan 29.
Ancel PY, Saurel-Cubizolles MJ, Di Renzo GC, Papiernik E, Bréart G.
Risk factors for 14-21 week abortions: a case-control study in Europe. The
Europop Group.
Hum Reprod. 2000 Nov;15(11):2426-32.