Trends in preterm birth in the Netherlands in 2011–2019: A population‐based study among singletons and multiples

Abstract

Introduction

Preterm birth (PTB) is the leading cause of infant mortality and morbidity worldwide. Rates of PTB in the Netherlands are declining, possibly due to the implementation of preventive strategies. In this study we assessed the overall trend in PTB rates in the Netherlands in recent years, and in more detail in specific subgroups to investigate potential groups that require scrutiny in the near future.

Material and methods

Based on the national perinatal registry, we included all pregnancies without severe congenital abnormalities resulting in a birth from 24 to 42 completed weeks of gestation between 2011 and 2019 in the Netherlands. We assessed PTB rates in two different clinical subtypes (spontaneous vs. iatrogenic) and in five gestational age subgroups: 24–27⁺⁶ weeks (extreme), 28–31⁺⁶ weeks (very), 32–33⁺⁶ weeks (moderate, 34–36⁺⁶ weeks [late] and, in general, 24–36⁺⁶ weeks [overall PTB]). Trend analysis was performed using the Cochran Armitage test. We also compared PTB rates in different subgroups in the first 2 years compared to the last 2 years. Singleton and multiple gestations were analyzed separately.

Results

We included 1 447 689 singleton and 23 250 multiple pregnancies in our study. In singletons, we observed a significant decline in PTB from 5.5% to 5.0% (p < 0.0001), mainly due to a decrease in iatrogenic PTBs. When focusing on different gestational age subgroups, there was a decrease in all iatrogenic PTB and in moderate to late spontaneous PTB. However, in spontaneous extreme and very PTB there was an significant increase. When assessing overall PTB risk in different subgroups, the decline was only visible in women with age ≥25 years, nulliparous and primiparous women, women with a medium or high socioeconomic status and hypertensive women. In multiples, the rate of PTB remained fairly stable, from 52.3% in 2011 to 54.1% in 2019 (p = 0.57).

Conclusions

In the Netherlands, between 2011 and 2019, PTB decreased, mainly due to a reduction in late PTB, and more in iatrogenic than in spontaneous PTB. Focus for the near future should be on specific subgroups in which the decline was not visible, such as women with a low socioeconomic status or a young age.

This study assessed recent trends preterm birth (PTB) rates in the Netherlands. Between 2011 and 2019, PTB rates decreased, mainly due to a reduction in late PTB, and more in iatrogenic than in spontaneous PTB.

Abbreviations

GA

gestational age

PTB

preterm birth

sPTB

spontaneous preterm birth

Key message.

In the Netherlands, the overall preterm birth rate in singletons decreased between 2011 and 2019. In multiples, the rate of preterm birth remained fairly stable.

1. INTRODUCTION

Preterm birth (PTB), defined as birth before 37 weeks' gestation, is a major contributor to perinatal mortality and morbidity, complicating over almost 15 million pregnancies worldwide. ¹ Preterm born children can suffer from long‐term physical and developmental impairment, such as neurocognitive, ophthalmological and respiratory morbidity. Consequently, PTB is one of the largest single contributors to the global burden of disease. ²

PTB is usually classified into two subtypes based on the onset of labor. Spontaneous PTB is defined as PTB after spontaneous contractions or preterm prelabor ruptured membranes (PPROM). Iatrogenic (or medically indicated) PTB is the result of an intervention for maternal or fetal problems, such as preeclampsia or fetal growth restriction. Especially the etiology of spontaneous PTB is complex and multifactorial.

Two previous studies described trends in PTB rates in the Netherlands, in both singletons and multiples. First, Schaaf et al. showed a significant decrease in the PTB rate from 6.4% in 2000 to 6.0% in 2007 in singletons, mainly driven by a decline in spontaneous PTBs. ³ Subsequently, van Zijl et al., using slightly different inclusion criteria to define the cohort (no congenital anomalies), reported a significant decline in PTB rate from 5.6% in 2008 to 5.3% in 2015. ⁴ This decline was a result of both spontaneous and iatrogenic declining PTB rates in singletons.

Reduction of PTB rates can be achieved by reducing general risk factors, such as smoking cessation and other lifestyle improvements. Governmental programs have have made a great effort to reduce smoking in the general population, and in pregnant women in particular. The Dutch Society of Obstetrics and Gynecology implemented the guideline on prevention of recurrent spontaneous PTB in 2007 which focused on the use of progesterone and/or cerclage in specific high risk subgroups. The use of aspirin in high risk women in the prevention of preeclampsia and fetal growth restriction has been implemented in the Netherlands in recent years. ⁵

There is a wide variation in PTB trends in European countries. ⁶ Understanding country specific differences in rising or declining trends could lead to information on strategies for prevention. Moreover, it is important to assess the effect of currently used preventive measures like progesterone and aspirin on the trend in PTB rates on a nationwide level. Trends in PTB rates in specific risk groups can point out which women do not benefit from current interventions, in order to focus on prevention in those groups in future years.

In this study, we assessed spontaneous and iatrogenic PTB rates from 2011 to 2019 in the Netherlands using data from the national perinatal registry. We did not include the last available year 2020 due to the COVID‐19 restrictions in this year. This could have influenced the PTB rate and is therefore analyzed in separate specific studies. ⁷ , ⁸ The aim of this study was to describe the recent trends in PTB in the Netherlands among singleton and multiple pregnancies, focussing on the different gestational ages and on subtypes of PTB. Furthermore, we compared rates of PTB in the last 2 years (cohort 2018–2019) compared to the first 2 years (cohort 2011–2012) in different risk groups.

2. MATERIAL AND METHODS

2.1. Study population

This study was a population‐based cohort study using data from the Dutch perinatal registry Perined from 2011 to 2019. ⁹ The Perined registry collects data on all births (live births and stillbirths) above 16 weeks of gestation and contains information on pregnancy complications, deliveries and neonatal hospital (re)admissions until 28 days after birth. The registry covers almost 98% of all births in the Netherlands. It is used primarily for an annual assessment of the quality indicators of obstetric care.

2.2. Inclusion and exclusion criteria

For this study, we selected all pregnancies resulting in birth between 24⁺⁰ and 42⁺⁶ weeks of gestation, from January 1, 2011 to December 31, 2019. For our main analysis, we excluded births at 22 and 23 weeks of gestation according to Delnord et al., advising to exclude pregnancies with gestational age (GA) <24 weeks when comparing PTB rates. ¹⁰ This is because of the impossibility to exclude terminations of pregnancy, which are not always as such registered in Perined. Pregnancies complicated by lethal or severe congenital abnormalities were excluded from analysis, since these cases have an intrinsic high risk of PTB and have a different clinical pathway. Lastly, we excluded all pregnancies with an antepartum fetal death, as they are not considered PTBs. We analyzed births with GA 20⁺⁰ and 23⁺⁶ weeks with spontaneous onset separately.

2.3. Outcome measures

The main outcome was PTB, defined as birth below 37 weeks of gestation. Gestational age was based on crown rump length measured during first‐trimester ultrasound. Secondary outcomes were rates of PTB between GA 34⁺⁰ and 36⁺⁶ weeks (late preterm), GA 32⁺⁰–33⁺⁶ (moderate preterm), GA 28⁺⁰–31⁺⁶ weeks (very preterm), GA 24⁺⁰–27⁺⁶ weeks (extreme preterm), stratified for spontaneous and iatrogenic start of labor. Spontaneous preterm birth (sPTB) included all births after spontaneous onset of contractions with or without prelabor rupture of membranes (PROM). Iatrogenic PTBs were all births after induction of labor or elective cesarean section.

2.4. Covariates

Continuous and categorical variables available in the Perined database were categorized in maternal and neonatal characteristics as follows: maternal age (< 25 year, 25–34 year, ≥ 35 year); parity (P0, P1, P2+); ethnicity (Western, Mediterranean including Turkish and Moroccan, other non‐Western ethnicity including Afican and South Asian); socioeconomic status (high [20%], medium [60%], low [20%]); Assisted reproductive technology (yes, no) hypertensive disorders, including pre‐eclampsia (yes, no); diabetes, including type 1, type 2 and gestational diabetes (yes, no); presentation (cephalic, noncephalic); mode of delivery (vaginal, instrumented, acute cesarean, planned cesarean, unknown); sex (male, female) and small for gestational age p10 (yes, no).

2.5. Statistical analyses

We reported on PTB rates in singleton and multiple gestations separately, because of their known differences in gestational age at birth and risk for PTB. We compared PTB rates for maternal and neonatal characteristics using univariate analyses with the chi‐square test.

First, to investigate whether there was a trend in PTB rate over time, we performed Cochran‐Armitage trend tests, with the year as the independent variable and PTB risk as the dependent variable. For both singletons and multiples this was performed for PTB <37 weeks and for the GA subgroups of PTB; late preterm, moderate preterm, very preterm and extreme preterm.

Second, we assessed rates of PTB in every risk group between two time cohorts: all births in the first 2 years (2011 + 2012) compared to all births in de last 2 years (2018 + 2019). We compared rates of PTB between the two groups using univariate analyses with the Student's t‐test and the Chi‐square test, where appropriate.

2.6. Additional analysis

In the Netherlands, neonates born at gestational age <24 weeks are not offered supportive care and therefore have a 100% mortality rate. Although excluded from our main analysis, they are regarded as PTBs. To assess whether there was a shift from immature births to extreme PTB, we defined a separate cohort consisting of birth between 20⁺⁰ and 23⁺⁶ weeks of gestation. We selected only births with spontaneous onset and performed a Cochran‐Armitage trend test to investigate whether there was a trend over time from 2011 to 2019.

Data were analyzed using SAS software (SAS Institute Inc. version 9.4).

3. RESULTS

3.1. Study population

In total, 1 539 103 records on child level were available in the Perined database. After predefined exclusion of cases, our final study population comprised of 1 447 689 women with a singleton pregnancy and 23 250 women with multiple pregnancies (Figure 1).

FIGURE 1.

Study flow chart.

3.2. Baseline

Table 1 provides an overview of PTB rates in risk groups for the whole cohort 2011 to 2019 in singletons and multiples separately. The overall PTB rate was 5.2% in singletons and 54.0% in multiples. Maternal risk factors for higher PTB rates include young maternal age <25 years, nulliparity, other non‐western ethnicity, assisted reproductive technology, hypertensive disorders, diabetes, and low socioeconomic status. This was similar for spontaneous and iatrogenic birth except for high maternal age ≥35 year which was an aditional risk factor for iatrogenic PTB. In Table S1, we summarized the numbers of PTBs per GA group according to subtype (spontaneous, iatrogenic, total) on which the trend analyses were performed.

TABLE 1.

Rates of preterm birth for maternal and neonatal characteristics by singleton and multiple birth for the whole cohort 2011–2019.

Characteristics	Singleton birth, N = 1 447 689	PTB 5.2%	p‐value	Spontaneous PTB 3.7%	Iatrogenic PTB 1.6%	Multiple births, N = 23 250	PTB 54.0%	p‐value	Spontaneous PTB 28.9%	Iatrogenic PTB 24.7%
Maternal age (years), SD	30.6 (4.8)			30.1 (5.0)	30.9 (5.2)	31.6 (4.8)			31.1 (4.8)	31.6 (5.0)
Maternal age (%)			<0.0001					<0.0001
<25	10.1%	6.2%		4.5%	1.7%	6.7%	62.1%		34.3%	27.8%
25–34	68.9%	5.1%		3.7%	1.5%	66.1%	54.4%		30.2%	24.2%
≥35	21.0%	5.2%		3.4%	1.9%	27.2%	49.5%		24.4%	25.1%
Parity (%)			<0.0001					<0.0001
P0	44.5%	6.7%		4.9%	1.9%	46.4%	60.9%		33.8%	27.1%
P1	36.0%	3.9%		2.7%	1.2%	35.1%	47.7%		25.0%	22.7%
P2+	19.5%	4.3%		2.8%	1.6%	18.5%	46.3%		23.8%	22.4%
Ethnicity (%)			<0.0001					0.0009
Western	79.7%	5.2%		3.6%	1.5%	80.3%	53.6%		29.1%	24.5%
Mediterranean	8.5%	4.9%		3.3%	1.6%	8.9%	50.6%		25.0%	25.6%
Other non‐Western	11.8%	6.1%		4.2%	1.8%	10.7%	56.1%		30.7%	25.5%
Socioeconomic status (%)			<0.0001					0.051
High	20.5%	4.8%		3.4%	1.4%	21.3%	52.0%		26.9%	25.2%
Medium	57.9%	5.2%		3.7%	1.6%	57.6%	54.0%		29.3%	24.7%
Low	21.6%	5.7%		3.9%	1.7%	21.1%	53.9%		29.8%	24.1%
ART	3.3%	8.1%	<0.0001	5.2%	2.8%	19.8%	52.6%	0.14	27.0%	25.5%
Hypertensive disorders a	6.2%	10.5%	<0.0001	2.8%	7.7%	3.0%	73.3%	<0.0001	18.5%	54.8%
Diabetes b	3.5%	6.1%	<0.0001	4.0%	2.1%	1.2%	55.1%	0.60	23.5%	31.6%

	Children, N = 1 447 689					Children, N = 46 470
Noncephalic	5.3%	16.0%	<0.0001	9.7%	6.2%	33.6%	54.9%	0.0046	27.7%	27.2%
Mode of delivery (%)			<0.0001					<0.0001
Vaginal	73.9%	4.4%		3.8%	0.6%	47.9%	54.2%		37.5%	16.6%
Instrumented delivery	8.1%	4.1%		3.4%	0.7%	6.1%	48.1%		29.1%	19.1%
Acute cesarean section	8.1%	7.1%		5.0%	2.1%	17.3%	59.4%		44.7%	14.7%
Planned cesarean section	7.5%	12.2%		0.0%	12.2%	26.1%	51.2%		0.0%	51.2%
Unknown	2.5%	8.1%		8.1%	0.0%	2.71%	55.7%		53.9%	1.8%
Male fetal sex (%)	51.5%	5.7%	<0.0001	4.1%	1.6%	51.1%	54.4%	0.07	29.1%	25.2%
Birthweight (grams), SD	3440 (548)			2468 (621)	2186 (772)	2403 (583)			2017 (574)	2199 (527)
SGA p10 (%)	10.4%	9.5%	<0.0001	3.6%	5.9%	32.1%	52.6%	<0.001	23.6%	29.0%

Abbreviations: ART, assisted reproductive technology; PTB, preterm birth; SD, standard deviation; SGA, small for gestational age.

^a Includes chronic hypertension, pregnancy‐induced hypertension and pre‐eclampsia.

^b Includes type 1, type 2 and gestational diabetes.

3.3. Singleton pregnancies

The rate of total PTB <37 weeks in singletons decreased from 5.45% in 2011 to 5.08% in 2019 (p < 0.0001). Although spontaneous PTB also showed a decrease (3.68%–3.62%, p < 0.043), the decline in total PTB was mainly a result of a decrease in iatrogenic PTBs (1.77%–1.34%, p < 0.001), see Figure 2. In absolute numbers, this means a reduction of around 200 PTBs a year in the Netherlands (1723 PTBs in 9 years).

FIGURE 2.

Singletons: rates of preterm birth (GA <37 weeks).

For spontaneous PTB, only late PTB (GA 34⁺⁰–36⁺⁶ weeks) showed a significant decline over the years (from 2.88% to 2.70%, p < 0.0001), see Figure 3. In extreme and very PTB with spontaneous onset, an increase was observed.

FIGURE 3.

Singletons: rates of spontaneous preterm birth per gestational age group.

For iatrogenic PTB, all four gestational age groups showed a decrease in the study period, all with a statistically significant trend, see Figure 4.

FIGURE 4.

Singletons: rates of iatrogenic preterm birth per gestational age group.

3.4. Multiple pregnancies

The rate of total PTB <37 weeks in multiples remained fairly stable from 52.3% in 2011 to 54.1% in 2019 (p = 0.57), see Figure 5. There was no trend in either spontaneous PTB (from 27.6% to 28.8%, p = 0.84) or iatrogenic PTB (from 24.7% to 25.3%, p = 0.39). The trends in late, moderate, very and extreme PTB in multiples are shown in Figures S1 and S2. We observed no trends in the spontaneous PTB gestational age subgroups. For iatrogenic PTB, an increase was seen in late PTB (18.9%–21.5%, p = 0.0002) whereas moderate, very and extreme PTB showed a decrease over time.

FIGURE 5.

Multiples: rates of preterm birth (GA <37 weeks).

3.5. Singletons and risk groups

We compared PTB rates in singletons for all maternal and neonatal characteristic groups between the first 2 years (cohort 2011–2012) and the last 2 years (cohort 2018–2019) (see Table 2). We observed a statistically significant decline in most risk groups for PTB. However, in the risk groups for PTB women <25 years of age (6.2% vs. 6.0%, p = 0.31), multiparous women P2+ (4.3% vs. 4.3%), women with Mediterranean ethnicity (4.9% vs. 4.7%, p = 0.32), women with low socioeconomic status (5.5% vs. 5.5%, p = 0.92), noncephalic presentation (16.1% vs. 15.5%, p = 0.16) and women who delivered small for gestational age babies (9.5% vs. 9.2%, p = 0.16), there was no statistically significant decline in PTB rate.

TABLE 2.

Comparison of maternal and neonatal characteristics and preterm birth rate in the first and last 2 years in singleton births.

Characteristics	2011–2012, N = 333 159	PTB rate 5.41%	2018–2019, N = 311 402	PTB rate 5.02%	p‐value <0.0001
Maternal age (years), SD	30.4 (4.9)		30.9 (4.7)		<0.001
Maternal age (%)
<25	11.6%	6.2%	8.6%	6.0%	0.31
25–34	67.9%	5.3%	69.4%	4.9%	<0.0001
≥35	20.5%	5.4%	22.1%	4.9%	<0.0001
Ethnicity (%)
Western	81.2%	5.3%	77.1%	4.9%	<0.001
Mediterranean	8.7%	4.9%	8.1%	4.7%	0.32
Other non‐Western	10.1%	6.7%	14.8%	5.8%	<0.001
Socioeconomic status (%)
High	20.4%	5.1%	21.0%	4.6%	<0.001
Medium	60.1%	5.5%	57.2%	5.0%	<0.001
Low	19.6%	5.5%	21.8%	5.5%	0.92
Parity (%)
P0	44.8%	6.9%	44.1%	6.3%	<0.0001
P1	35.7%	4.1%	35.8%	3.8%	0.0008
P2+	19.4%	4.3%	20.0%	4.3%	0.73
ART	3.6%	8.4%	1.9%	7.1%	0.002
Hypertensive disorders a	6.7%	10.7%	5.6%	9.9%	0.0047
Diabetes b	2.4%	6.4%	4.2%	5.6%	0.02
Cephalic
Noncephalic	5.2%	16.1%	6.2%	15.5%	0.16
Mode of delivery (%)
Vaginal	73.2%	4.5%	74.1%	4.2%	<0.0001
Instrumented delivery	9.5%	4.2%	7.0%	3.7%	0.0016
Acute cesarean section	8.8%	7.3%	7.6%	6.6%	0.0048
Planned cesarean section	6.9%	14.4%	7.5%	10.1%	<0.001
Unknown	1.7%		3.9%
Male fetal sex (%)	51.4%	5.8%	51.6%	5.5%	<0.0001
Birthweight (grams), SD	3451 (553)		3440 (542)
Small for gestational age (%)	10,5%	9.5%	10.3%	9.2%	0.16

Abbreviations: ART, assisted reproductive technology; PTB, preterm birth; SD, standard deviation; SGA, small for gestational age.

^a Includes chronic hypertension, pregnancy‐induced hypertension and pre‐eclampsia.

^b Includes type 1, type 2 and gestational diabetes.

3.6. Births 20⁺⁰–23⁺⁶ weeks

A total of 1500 singleton pregnancies ended in a spontaneous PTB between 20⁺⁰ weeks and 23⁺⁶ weeks, 0.104% of all singleton births. Trend analysis showed that the rate in these PTBs remained the same over the years, see Figure S3. In absolute numbers, a decrease was seen from 172 births in 2011 to 156 births in 2019.

4. DISCUSSION

Our study showed a decrease in both spontaneous and iatrogenic PTBs <37 weeks in singletons from 2011 to 2019 in the Netherlands, mainly driven by a decline in moderate to late PTBs. When focusing on different gestational age subgroups, we showed that very PTB and extreme PTB with iatrogenic onset decreased over the study period, whereas very PTB and extreme PTB with spontaneous onset increased. In multiples, the rates of PTB <37 weeks remained fairly stable over the years.

The results of the trend in PTB rate in the Netherlands in this study are concordant with the decrease in PTB rates in singletons shown by Schaaf et al. and van Zijl et al. from 2000 to 2015. ³ , ⁴ Although both studies used slightly different inclusion criteria, an overall reduction from 6.4% to 5.0% was observed over 20 years. The Europeristat Report from 2022 presented heterogeneous patterns of changes between 2015 and 2019, with both declining and increasing rates of PTB in European countries. ⁶ In the United States, trends from 2005 to 2012 show a decline in PTB rate in both spontaneous and iatrogenic preterm deliveries among singletons, ¹¹ although more recent reports suggest a stabilization of the PTB rate at around 10%. ¹² , ¹³

Although this study did not assess causality, the decline in overall sPTB and in spontaneous birth from 32 week onwards may be attributable to both lifestyle improvements and the implementation of preventive interventions that have been shown to successfully lower the incidence of sPTB. For instance, progesterone therapy as a component of the 2007‐instituted national policy for the “prevention of recurrent spontaneous preterm birth”. Additionally, at this time, screening for short cervical length in both low and high risk women was started in the Netherlands. Regarding lifestyle, the rate women who smoke during pregnancy, a major risk factor for PTB, is in steady decline in the last decade. ¹⁴

Our additional analysis on sPTB between 20⁺⁰ and 23⁺⁶ showed that the absolute lower number of sPTB between 2011 and 2019 almost equals that of the increase in sPTB between 24⁺⁰ and 27⁺⁶ weeks. One could hypothesize that due to medical interventions, there is a shift from birth <24 weeks to PTB at later gestational ages.

All gestational age subgroups in our study showed a decrease in iatrogenic PTB. Most iatrogenic PTBs follow the need for delivery in pre‐eclampsia or fetal distress, usually due to fetal growth restriction. Both the introduction of aspirin use during pregnancy in high risk women and smoking cessation policies may have influenced the prevalence of these syndromes or their disease course beneficially. ¹⁵ , ¹⁶ More importantly, the decrease observed in our study could be the result of a more expectant approach in pregnancies complicated by pregnancy induced‐hypertension, pre‐eclampsia or fetal growth restriction. ¹⁷

Not all risk groups exhibit the same share in the overall decline in PTB <37 weeks in our study. Both maternal young age and low socioeconomic status are known risk factors for PTB ¹⁸ , ¹⁹ and these groups did not show a significant reduction over the years. The prevention programs and medical intervention apparently do not reach these women or are unsuccessfull, indicating they need extra attention in future care. The PTB rate among Mediterranean women (Moroccan & Turkish ethnicity, the two biggest minorities in the Netherlands) is lower than women with West‐European ethnicity. To demonstrate a significant decline in an already low risk group is therefore challenging.

The number of multiple pregnancies has been steadily in decline in the Netherlands due to guidelines on single embryo transfers in in vitro fertilization treatment. ²⁰ Although we observed a decline in absolute PTBs in multiples, the PTB rate per 100 birth remained stable. Few studies have described PTB trends in multiples. In Victoria, Australia, a substantial increase in PTBs was seen in multiples between 2007 and 2017: from 52% to 69%, mainly driven an in increase in late and iatrogenic PTB. ²¹ van Zijl et al. reported a statistically significant increase in PTB rate in multiples from 2008 to 2015 in the Netherlands, whereas in our cohort, the increase was not statistically significant. ⁴ Although we found a decrease in very PTB, our results confirm that we have not been able to successfully reduce the risk for PTB <37 weeks in multiple pregnancies in the Netherlands. Additionally, guidelines from 2011 recommend to induce monochorionic twins between 36 and 37 weeks, rather than the prior recommendation of 37 weeks. ²² This might have led to an increase moderate to late PTB with iatrogenic onset shown in this study. Unfortunately, information of chorionicity was not available in the national perinatal registry.

A strength of this study is the nearly complete perinatal registry database of all births in the Netherland; the registry is almost 98% complete, allowing us to investigate nearly 1.5 million births. Second, we differentiated between singletons and multiples, and assessed different subtypes in onset (spontaneous vs iatrogenic) and gestational age types, each with possibly different etiologies. We chose to exclude all pregnancies GA <24 weeks, to make a fair comparison with other studies. Although data from birth 20⁺⁰ to 23⁺⁶ are less reliable due to the inclusion of terminations of pregnancy, we believe that our analysis in this group provides insight in PTBs, since we only include deliveries with spontaneous onset.

A limitation of the national perinatal registry is the lack of (high quality) information on obstetric history including a history of PTB, body mass index and smoking. PTB rates in these important risk groups could not be provided. Furthermore, hypertensive disorders and diabetes (all types) are known to be underreported in the Perined registry. This is also demonstrated by the low incidences in our cohort.

The overall PTB rate is mainly driven by the rate of moderate to late PTB (GA 32⁺⁰ weeks–36⁺⁰ weeks), since this groups makes up the majority (85%) of all PTBs. Although the overall PTB rate in the Netherlands decreased, it badly reflects the trends in very and extreme PTB (GA <32⁺⁰ weeks), the group with the most unfavorable clinical outcome. This study highlights the importance of assessing trends not only in overall PTB, but also according to clinical subtypes (spontaneous vs. iatrogenic) and in different GA subgroups. The increase in spontaneous very and extreme PTB, in particular, warrants our full attention. Apparently our current interventions are not able to prevent these births. Further research is necessary to better understand the many causes of PTB and how they affect PTB at various gestational ages groups.

5. CONCLUSION

Our study reports a decline in singleton PTBs in the Netherlands from 2011 to 2019, especially in PTBs with iatrogenic onset. The trend in spontaneous PTB <32 weeks increased. In multiples, PTB rates remained stable over the recent years. Future public health intervention should focus on risk groups that do not exhibit the same share in the overall the decline, such as women with a low socioeconomic status or a young age.

AUTHOR CONTRIBUTIONS

Design: ACJR, JK. Analyses: ACJR. Writing: JK. Supervision: MAO, ACJR. JK, ACJR, CR, AA‐H and MAO contributed to the interpretation of the analyses and writing of the manuscript. All authors approved the final manuscript.

FUNDING INFORMATION

None.

CONFLICT OF INTEREST STATEMENT

The authors declare no conflicts of interest.

Ethics statement

The data used in this study were part of an anonymized database of routinely collected data and therefore approval of an Ethics Review Board was not necessary. Permission was given by Perined, the Dutch perinatal registry, to use records and analyze data for the purpose of this study (approval no. 21.100, dated August 25, 2021).

Supporting information

Figure S1.

Figure S2.

Figure S3.

Table S1.

Klumper J, Ravelli ACJ, Roos C, Abu‐Hanna A, Oudijk MA. Trends in preterm birth in the Netherlands in 2011–2019: A population‐based study among singletons and multiples. Acta Obstet Gynecol Scand. 2024;103:449‐458. doi: 10.1111/aogs.14684