In the overall population, the trends in gestational age and BW show a significant increase in preterm birth and low BW, a reduction in term and postterm births and a stable level of birth weights above 4000 grams.
Preterm births increased most among births before 32 weeks, and LBW births in the category with BW < 1500 g. These trends deserve special attention because these groups of live births have higher known risks of mortality, morbidity, and sequelae [7, 17, 18], and also because these trends were most pronounced during the most recent period. Another trend to consider is the increase in multiple births because this group continues to have a high prevalence of prematurity, exceeding 50% during the last period
These trends seem to correspond to a real change; they have been obtained from a database where the records of the variables of interest can be considered good and its exhaustiveness has been observed through the three periods, expressed by a very low proportion of missing values. The observed changes were significant and consistent across all three periods, they involved the entire population of births and affecting singletons and multiple births. The measurements recorded are also highly biologically plausible, in terms of the very small proportion of misclassifications and values ≥44 weeks. Similarly, the trend in birth weight was consistent with that of gestational age.
Although the aim of this study is descriptive and not explanatory, the principal limitation of our research, like all research from general population databases, was the lack of more precise information to enable us to answer more specific questions .
Therefore, we lack more precise information of the method used to asses individual GA. Thus we cannot differentiate between those cases evaluated with one method or the other nor those GA corrected by USN and those not.
We miss as well an association with the Stillbirth registry, which prevented us from obtaining complete information about changes in GA and BW for those who die before birth.
Despite such limitations, it appears useful to focus the discussion on the possible effects on these trends of changes in obstetric and perinatal management that are capable of modifying the distribution of births at the population scale to shorten the duration of gestation. This focus should help to guide the formulation of basic hypotheses for future research.
The potential influence of some factors that could modify the distribution of births
The World Health Organization explains that the increase in register of live birth is associated to the possibilities of survival where the birth took place . Then we may reasonably assume that the national polices for perinatal care in all country, according to a plan for the regionalization (since 1990), increased the probability of early viability, and consequently, increased the recording of extremely preterm live births since 1990. Due to a lack of link with the register of stillbirth, we cannot value this possibility.
Similarly, the misclassifications and missing values should have been progressively able to integrate the group of preterm births, but their number always remained low and relatively stable, and most of the births with missing values for GA had a BW corresponding to term births. The births with missing values for weight had more preterm deliveries, but this category of birth has diminished markedly since 2001 and could not contribute to recent trends.
The role of other more widespread factors, such as the changes in maternal age signalled in demographic reports , remains to be determined, especially the teenaged and older mothers. Studies in Chile report an increase in both groups during the observed period  and evidences show that these mothers are at high risk of give birth a child with very preterm birth and low birth weight .
Advanced maternal age may be a risk factor, both for the rates of multiple pregnancies but also in terms of greater recourse to treatment for infertility. It has been observed in France that from a quarter to a third of multiple births are related to increase of maternal age, and more than 30% to treatment for infertility .
Nonetheless, the increase in multiple births does not appear to be linked to procedures of fertilisation or assisted reproduction, for the elevated cost of these procedures makes them still quite rare. According to the reports of the Latin American Network for the assisted reproduction (REDLARA) only 480 births of this type was born in 2008, 23% of them twin births .
The possible influence of LMP and USN as methods to estimate GA
It is well known that the choice of method for estimating gestational age can influence perinatal outcomes . This should not be surprising: the methods rely on different parameters. LMP measures the duration of gestation while USNF is based on fetal anthropometric measurements .
LMP is greatly affected by the individual characteristics of mother and fetus, and the gestational age tends to be greater, while ultrasound classifications consistently skew to younger ages, and tend to predict shorter pregnancies.
As for the effect of these two methods on perinatal outcomes, divers studies show that LMP is associated with a higher incidence of adverse outcomes, including preterm, postterm, and growth fetal restriction [26–29]. Ultrasound estimate also appear to correlate with a greater incidence of premature births than LMP, decreased birth weight as well as a clear reduction of exceeding 41 weeks [29–31].
Ultrasound also may diagnose fetuses smaller than the mean or having growth restriction as having less gestational age. Conversely, fetus determined to be oversized for its gestational age may be classified to a more advanced gestational age [27, 32].
In sum, it appears that the use of one single method (US or LMP) has strengths and limitations. We could expect that we can obtain better estimations if the two methods are considered and particularly if there is concordance between their results [26, 30].
In Chile, the studies that have assessed the fetal ultrasound program for the first trimester observed good concordance between the date of the last menstrual period and the ultrasound date, and this concordance has remained relatively stable over time (61.9% in 1994 and 65.6% in 2001) . Other studies have confirmed these findings (weighted Kappa: 0.64) .
In our study it seems that the choice of the one or the other method did not change the increasing trends in the rate of preterm.
On the other hand, the increased use of ultrasound estimates could have reduced the incidence of errors and missing values as well as in the values exceeding 41 weeks [34, 35].
The decrease in births between 37 and 41 weeks, with a large reduction in births after 41 weeks, may be related to increased medical intervention during pregnancy
Since the 1990s, nationwide clinical guidelines based on high risk approach have been implemented to improve maternal and perinatal health . The large reduction in births after 41 weeks might thus be associated, on the one hand, with first-trimester fetal ultrasonography, and on the other hand, with the termination of at-risk pregnancies.
At the same time, between 1990 and 2000, prenatal care coverage rose from 85% to 91.4%, and the percentage of women who began prenatal care before 20 weeks of gestation rose from 74% in 1994 to 86% in 2000. All these changes probably increased the opportunities for screening for disorders and preventing complications.
In this context, the observed trends may also express medical practices that were more active in the face of maternal or fetal risks and led to more frequent recourse to caesarean birth or induction of labour.
This is the case for the deliveries that, according to national guidelines, are induced from week 41 to diminish fetal risk; this guideline may also help to explain the very low number of postterm infants, as observed elsewhere, as well as the stability of birth weights exceeding 4000 g .
Thus, the underlying reason for these preterm birth trends must be considered in future research that must distinguish between spontaneous preterm deliveries and those considered medically indicated.
Observations in 13 European countries show that the countries with the highest rates of induction of labour also have the lowest rates of postterm births . Similarly, researches in North America  and South America [38, 39] show that excess rates of caesarean deliveries and induction of labour are important contributors to the increase in preterm birth and to the reduction of birth weight.
Several authors have shown that even when taking into account the role of other factors including maternal age, plurality [40, 41] or the method for calculating gestational age , the effect of the obstetric interventions is clearly of major importance.
According to a trend study conducted from the perinatal information system for the countries of Latin America and the Caribbean (SIP), 40% of preterm births were associated with medical interventions . Deliveries involving induction of labour or elective caesareans have increased over the past 20 years from 10% in 1985 to 18.5% in 2005, accompanied by an increase in preterm and very preterm births. The countries most involved in this increase were Argentina, Brazil and Chile .
The caesarean rates are especially high in Chile ; during the observed period the national caesarean rate was reported to be 40%: it fluctuates around 30% in public hospitals, and around 50% in private hospitals, exceeding 60% in some facilities [43, 44]. The effect of obstetric interventions on mothers and babies has been evaluated [39, 43], and studies have shown, for example, that the decline in the category of weight greater than 4000 g is due to deliveries induced before completion of the cycle of major weight gain that occurs after week 37 and can reach 600 g .
The impact of these wide-scale medical interventions on mothers is both visible and cumulative. Accordingly, a woman who has already had a caesarean delivery in Chile has a risk of a second caesarean delivery 22 times higher than a primipara .
From a comparative point of view, these trends are consistent with those described throughout both North and South America
In America, the largest increase in preterm birth rates from high-income countries has been reported in the United States. Increase is also important in some medium-income countries, including Brazil. The prevalence rate of PTB in both countries is around 12% .
This research shows that in Chile, the overall rate of preterm births and low birth weight may be considered relatively low; but there are specific trends to be considered, such as the increase of newborns with less than 32 weeks of GA. This population is at high risk of morbidity and sequelae; might require specialized management and furthermore could have a significant impact on the public health of the country.
Changes such as those observed in these two perinatal outcomes over a considerable period of time could provide information that can be used in the short or long term in several domains of public health as follow up care, financial planning etc. Even more when these current trends could be prolonged in the absence of other targeted prevention policies.