Introduction

In recent years, the topic of changing patterns of maternal and family characteristics has been a popular theme in public health (OECD 2011). In developed countries, it is generally accepted that women are giving birth at a later age, have a higher BMI and are less likely to smoke (Australian Institute of Health and Welfare 2021b; Morgan et al. 2014; Azagba et al. 2020). Public discourse around this topic is voluminous, with articles published in major international titles, such as Forbes (Stahl 2020), The Atlantic (Khazan 2018), The Guardian (Roberts 2018) and The New York Times (Tavernise et al. 2021). However, there is a dearth of peer-reviewed literature examining these trends across time. Australia is a developed country, with universal health care, low infant mortality (OECD n.d.) and perinatal data collections that capture virtually all births (Queensland Health 2017). Therefore, it is a useful case study in assessing maternal and neonatal trends over time.

Within the Australian context, various government organisations have looked at changes in maternal and neonatal patterns across time. The Australian Institute of Health and Welfare issues an annual report titled Australia’s mothers and babies, which summarises maternal and neonatal data for any particular year (Australian Institute of Health and Welfare 2021a). The most recent report showed that mothers are smoking less than the previous year, but with only small changes in the rate of low birthweight between 2009 and 2019 (Australian Institute of Health and Welfare 2021a). Other organisations such as the Australian Institute of Family Studies, publish reports on maternal trends of relevance to their core business. The Families Then & Now: Having children report showed that maternal age at first child has increased (Qu 2020). The report found that, in 1981, 15% of first-time mothers were aged over 30 years compared with 49% in 2017. These reports, while certainly helpful at highlighting trends, are limited in their temporal scope, and tend to take a broad approach rather than assessing trends related to specific outcomes, such as birthweight. Low birthweight (<2500 grams) is a key indicator for child health (Australian Institute of Health and Welfare 2018) yet the proportion of low birthweight babies has remained relatively stagnant over recent years (Australian Institute of Health and Welfare 2022).

It has previously been shown that maternal pre-pregnancy body mass index (BMI), gestational diabetes, and pre-existing diabetes are associated with age-standardised birthweight in multivariable regression models of a Queensland population (Vilcins et al. 2020). However, these models of association do not show how the trends are changing over time. Routine data collections, such as perinatal data collections, provide a useful tool for assessing long-term trends in maternal and neonatal health. These collections capture a broad range of information and tend to capture most of the eligible population. The aim of this study is to assess the trends in birthweight and maternal characteristics across a 19-year period using descriptive statistics. To address this aim, we will explore maternal and neonatal trends in three sections. The first section presents neonatal trends between 2000 and 2019, namely birthweight, birth length and gestational age. The second section presents trends in maternal characteristics over the same time period. We have selected those characteristics that have previously been shown to be important predictors of birthweight in this population (Vilcins et al. 2020). Finally, we explore the trends in birthweight across time, accounting for the maternal characteristics explored in the previous section. To aid in interpretation, we present our findings in figures in addition to text summaries.

Methods

The Queensland Government Perinatal Data Collection (PDC) is a comprehensive collection of birth records maintained for virtually all births in Queensland, Australia. Data were obtained for the period of 1 January 2000 to 31 December 2019. The total number of records for this period was 1,166,055. The population of interest were live born, singletons with no congenital anomalies. Records were removed if they were multiple births (n = 37,209), stillbirth (n = 7544), or had a known or suspected congenital anomaly (n = 67,471). Records were excluded if birthweight (n = 182) or gestational age (n = 68) were missing, birthweight was less than 100 g (n = 149), or where baby sex was indeterminate (n = 141).

Age standardised birthweight categories were created using the ‘childSDS’ package (Vogel 2017). Briefly, birthweight, gestational age and child sex were used to calculate an age- and sex-standardised percentile ranking based on World Health Organisation reference values (Vogel 2017). Records below the 10th percentile were allocated as small for gestational age (SGA) and above the 90th percentile as large for gestational age (LGA), which follows the Australian definitions for this outcome (Australian Institute of Health and Welfare 2021a). These can be interpreted as meaning babies born SGA are in the bottom 10% of the sex-specific references and babies born LGA are in the top 10% of the sex-specific references. Extremely preterm babies (below 22 weeks) were not categorised due to software limitations (n = 437). Maturity at birth was categorised according to the guidelines of The American College of Obstetricians and Gynecologists as well as World Health Organization – where <28 completed weeks is extreme preterm, 28–32 weeks very preterm, 32–37 weeks preterm, 37–38 weeks early term, 39–40 weeks full term, 41 weeks late term, 42+ weeks post-term (The American College of Obstetricians and Gynecologists Committee on Obstetric Practice Society for Maternal-Fetal Medicine 2013; World Health Organization 2018). Maternal BMI was categorised following the US Centres for Disease Control and Prevention (CDC) guidelines (CDC 2021).

We used descriptive statistical methods to show the trend across the study period. Plots and summary tables were chosen to visually represent these trends. All work was performed in R statistical program (R Core Team 2016).

Results

Characteristics of the study population

In total, 1,059,117 records were included in the analysis. The mean birthweight across the study period was 3410.5 grams (SD 552.6) and the majority of babies were born at an appropriate size for their gestational age (81%). Most babies were born after 37 weeks gestation (93.6%), with the mean gestational age in the cohort of 38.9 weeks. Mothers were predominately non-smokers (84.5%), with 50.7% of women in a healthy BMI range (see Table 1).

Table 1 Characteristics of the study population
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Trends in neonatal outcomes

Context

In this section we explore the following neonatal outcomes: birthweight, age-standardised birthweight, birth length and gestational age. Each outcome is presented as a written summary and plotted across each year of the study period.

Findings

Mean birthweight, for all infants regardless of gestational age, in Queensland has been steadily declining over time; from a high of 3431 g in 2001 to the lowest mean weight of 3371 g in 2019 (the most recent year in this analysis). See Fig. 1. However, when stratifying the by preterm status, the trend downwards in birthweight was less pronounced and only seen in full term babies (> 37 weeks completed gestation) (see Supplementary materials, Fig. S1).

Fig. 1
figure 1

Mean (SE) birthweight by year in Queensland, Australia from 2000 to 2019

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The rate of small for gestational age babies has decreased across the study period, from a peak of 5.78% of live, singleton births in 2002 to 3.99% of similar births in 2019. The rate of large for gestational age births was higher in 2019 compared with 2000; however, this increase has not been linear (see Fig. 2).

Fig. 2
figure 2

Trends in age-standardised birthweight outcomes from 2000 to 2019 in Queensland, Australia

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Mean gestational age decreased by around 3.5 days, from 39.1 weeks in 2000 to 38.6 weeks in 2019 (Fig. 3). Most babies were born at early term (27%) or full term (53%). There were changes in the gestational age categories across the period. Rates of early term births increased across the period, from 21.6% in 2000 to 34.4% in 2019. During the same period, the rates of full term and late term birth dropped by around 5.6% and 6.3%, respectively (Fig. 4).

Fig. 3
figure 3

Mean (SEM) gestational age in completed weeks, by year in Queensland, Australia, between 2000 and 2019

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Fig. 4
figure 4

Proportion of births in each term category per year, for births in Queensland, Australia, between 2000 and 2019

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Mean birth length decreased steadily over the period (Fig. 5). In 2000, the mean length at birth was 51.1 cm, while in 2019 there was a small reduction to a mean of 50.8 cm. Similarly to birthweight, this trend differs by preterm status. For preterm babies, there was a small increase in birth length across time (see supplementary materials, Fig. S2).

Fig. 5
figure 5

Mean (SEM) birth length for a cohort of Queensland babies between 2000 and 2019

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Trends in maternal characteristics

Context

In this section we explore the following maternal characteristics: body mass index, age, smoking status, pre-existing and gestational diabetes, mothers who identify as Indigenous, and the gestational age of baby when mother first presented for antenatal care. We present the trends across the study period in both written summary and plots.

BMI

Higher maternal pre-pregnancy BMI has been highlighted as a concern for maternal and neonatal health (Leddy et al. 2008). There were small, non-linear shifts in mean BMI by year. The highest mean BMI value was 26.0 in 2019, while the lowest was 25.4 in the years 2012 to 2015. Small, non-linear, changes across time are also seen for BMI categories (Fig. 6). The proportion of underweight mothers rose slightly across the period from 4.3% in 2007 to 5.1% in 2019. Similarly, the proportion of obese mothers rose from 20.1% in 2007 to 22.3% in 2019. The proportion of overweight mothers declined, from 26.8% in 2007 to 24.3% in 2019.

Fig. 6
figure 6

Proportion of births by maternal BMI category for births in Queensland, Australia, between 2000 and 2019

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Mean pre-pregnancy BMI increased in a non-linear fashion with age (Fig. 7). Within the age categories, mean BMI stayed relatively stable across the cohort period.

Fig. 7
figure 7

Mean (SEM) pre-pregnancy BMI, by maternal age category in Queensland, Australia, between 2000 and 2019

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Maternal age

The pattern of maternal age across time showed several interesting shifts (Fig. 8). At the extremes, there was a slight decrease of babies born to women under 15 years and a slight increase in women birthing at 45 years or older. There was a decline in the proportion of women aged 15–19 years and a decline in the proportion of births to women aged 20–24 years. The greatest proportion of births were to women aged 25 to 34 years. In the 25–29 year age bracket, there was a decline in the proportion from 32.4% in 2000 to 28.1% in 2019. In contrast, the proportion of births to women aged 30–34 years increased, from 28.1% to 32.8%. The strongest change occurred in the 35–39 years age bracket. There was an increase from 12.5% to 18% of births to women in this age bracket. There was also a small increase in the proportion of births to women aged 40–44 years, from 2.1% to 3.6%. Mean maternal age increased, from 28.4 years to 30.0 years.

Fig. 8
figure 8

Proportion of births by maternal age group for births in Queensland, Australia, between 2000 and 2019

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Maternal smoking

Maternal smoking had a consistent decline across the study period. In 2005, 20.6% of women smoked during their pregnancy compared with 11.6% of women in 2019. Younger women were more likely to smoke; 37.1% of women aged between 15–19 years smoked during their pregnancy, compared with 14.9% of women aged 25–29 years (Fig. 9).

Fig. 9
figure 9

Proportion of maternal smoking by age category for births in Queensland, Australia, between 2000 and 2019. This figure has <15 years and >45 years removed due to low numbers

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Diabetes status

The proportion of women with pre-existing diabetes rose steadily across the study period but remained low at 0.9% of births in 2019. The rate of gestational diabetes rose slightly between 2000 and 2010, but then started to increase more rapidly. In 2000, 3.9% of women developed gestational diabetes, but by 2019 the proportion had increased to 13.1% (Fig. 10). This rise may be partially explained by the rise in mean BMI over time, but the trend of a sharp increase from 2010 onwards does not match with the small and non-linear rise in BMI.

Fig. 10
figure 10

Proportion of births by diabetes status for births in Queensland, Australia, between 2000 and 2019

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Indigenous status

Across the study period, births to women who identified as Aboriginal and/or Torres Strait Islander increased slightly. In 2000, 5.8% of women were indigenous which increased to 7.5% in 2019. Indigenous women tended to be younger, with 49.5% of women under 25 years of age, compared with 20.0% of non-indigenous women.

First presentation for antenatal care

Data for gestational age at first visit were collected from 2009 onwards, and across the study period there has been a fairly linear trend down in gestational age at first presentation for antenatal care, from a mean of 13.8 weeks in 2009 to 10.3 weeks in 2019. Indigenous women tended to present later in their pregnancies (14.2 weeks compared with 11.7 weeks for non-Indigenous women). Women over 25 years presented before 12 weeks gestation, while younger women presented at 13.2 weeks for 20–24 years, 14.5 weeks for 15–19 years, and 19.3 weeks for those under 15 years. Smokers had a mean gestational age of 14.4 weeks compared with non-smokers at 11.4 weeks. Women with pre-existing diabetes tended to present slighter earlier for antenatal care compared to non-diabetics, at 10.6 and 11.8 weeks, respectively (see supplementary materials, Figs. S3 – S8).

Trends in birthweight, by maternal characteristics

Context

In this last section we show the trends in birthweight across the study period, accounting for the selected maternal characteristics.

Maternal BMI and birthweight

Mean birthweight increased across maternal BMI categories (Fig. 11). Women who were obese had babies with a mean birthweight 140 grams higher compared with babies born to women with a normal BMI (see supplement, Fig. S10). Women who were underweight, birthed babies who were 201 grams lighter than normal weight women. Of babies born to overweight and obese women, 17.0% and 23.2% were born LGA, respectively. The proportion of SGA babies decreased as maternal BMI decreased, with 8.4% of babies born to underweight mothers SGA compared with 4.5% to normal weight mothers.

Fig. 11
figure 11

Mean (SEM) birthweight (grams) by maternal BMI category in Queensland, Australia, between 2000 and 2019

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Maternal age and birthweight

The pattern of birthweight across maternal age categories took an inverted U-shaped distribution (Fig. 12). Women under 20 years of age or ≥ 40 years had lower mean birthweights compared with women aged 20–39 years. When comparing age-standardised birthweight, women aged 40 or older had similar proportions of SGA and LGA babies as women aged between 20–39 years. In contrast, younger mothers were far more likely to deliver a SGA baby, with 7.3% and 11.1% of babies born to women aged 15–19 years and <15 years, respectively, being SGA.

Fig. 12
figure 12

Mean (SEM) birthweight (grams) by maternal age group in Queensland, Australia between 2000 and 2019

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While mean birthweight by age category has trended down across time, in line with the general trend in birthweight, most age bands have stayed fairly stable with minor downward trends. Mean birthweight of babies born to mothers aged 40–44 experienced the largest decrease, with a mean birthweight of 3372 g in 2000 compared to 3298 in 2019, a 74 gram decrease (Fig. 13).

Fig. 13
figure 13

Mean (SEM) birthweight (grams) by maternal BMI category in Queensland, Australia, between 2000 and 2019. This figure has <15 years and >45 years removed due to low numbers

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Maternal smoking and birthweight

Women who smoked during pregnancy had lower birthweight babies compared with non-smoking women (Fig. 14). The mean birthweight of babies born to smokers was 3219 grams, compared with 3442 grams for non-smoking women. There was an associated increase in the proportion of SGA babies, rising to 8.9% of babies born to smoking women compared with 3.4% in non-smokers. The gap between mean birthweight remained steady across the study period.

Fig. 14
figure 14

Mean (SEM) birthweight (grams) by maternal smoking and year of birth in Queensland, Australia

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Pre-existing and gestational diabetes and birthweight

There was little difference in the mean birthweight of babies born to mothers with gestational diabetes (3377 grams) or pre-existing diabetes (3446 grams) compared with mothers without diabetes (3413 grams). However, differences were present for age-standardised birthweight. The proportion of LGA babies for women without either type of diabetes is 14%. The proportion of LGA babies born to women with gestational diabetes rose to 20% and for women with pre-existing diabetes the proportion was much higher at 47%. Prior to 2005, the mean birthweight of babies born to women with gestational diabetes was slightly higher than non-diabetic women, but from 2005 onwards, there is a steep decline in the mean birthweight (Fig. 15). Following the rise in rates of gestational diabetes reported earlier, there was an increase in the proportion of babies born both LGA and appropriate for gestational age to these mothers, while rates of SGA stayed steady (Fig. S11 in Supplementary materials).

Fig. 15
figure 15

Mean (SEM) birthweight (grams) by gestational diabetes status and year in Queensland, Australia

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Maternal Indigenous status and birthweight

Babies born to Indigenous women tended to be 171 grams lighter than babies born to non-Indigenous women, on average. The proportion of SGA births for Indigenous women were 7.9% compared with 4.2% in the non-Indigenous women. There was also an associated decrease in the risk of LGA births, with 11.8% of births LGA for Indigenous women compared with 14.7% in non-Indigenous women. The mean birthweight across the study period fluctuated, reaching a low point of 3214 grams in 2005 and returning to 3266 grams in 2019. This trend is different to the trend seen across the study period for non-indigenous women (Fig. S9, supplementary materials).

Discussion

In our analysis of birthweight in Queensland Australia, a developed nation with universal health care, we found that across the study period the mean birthweight of babies has seen a small decrease. When stratified by maturity at birth, this trend was only seen in the full-term babies, and preterm babies experienced a small increase in mean birthweight. Maternal smoking, BMI and Indigenous status were found to influence the patterns in mean birthweight seen in this cohort. Understanding trends in birthweight can help us identify interventions that may be useful in preventing adverse birthweight outcomes, especially if confirmed with studies using statistical models that address confounding to test the association. Some interesting trends highlighted in this work are discussed.

As is commonly posited, mean maternal age rose across the study period. As discussed in the introduction, this topic is popular as a source of news fodder, but little information about the reasons why women delay childbearing is available in the scientific literature. Published research has tended to focus on trends (Solanke et al. 2019; Matthews and Hamilton 2009) or on a woman’s understanding of the risks of delayed childbearing (Matthews and Hamilton 2009; Delbaere et al. 2020). The trends in our descriptive study show that across a 19-year period there was a decrease in the proportion of births to women aged ≤ 29 years and an increase in the proportion for each age bracket above 30 years. A recent analysis of births in the United States support these changes, finding a 194% increase in women giving birth over 40 years of age over a three decade period (1989–2018) (Bornstein et al. 2020). What does seem to be clear is that increased maternal age is associated with a reduction in fertility, poorer neonatal outcomes and increased risk of maternal complications (Attali and Yogev 2021; Lean et al. 2017). The inverted-U shaped association between maternal age and birthweight has been previously found in other studies (Aras 2013; Wang et al. 2020), including a study that looked at trends within families to address unmeasured confounded at the family level (Goisis et al. 2017).

Maternal weight may have an important role on subsequent infant weight. In our trend analysis, underweight mothers consistently had mean birthweights lower than the other BMI categories. It has previously been shown that the mean birthweight of babies born to underweight women is 167 g lower compared with normal weight women, and had almost double the prevalence rate for SGA births (Jeric et al. 2013). There may be a sex-specific effect, with a study of underweight Japanese women finding that maternal underweight was associated with increased odds of SGA in female neonates only (Kasuga et al. 2019), although more work is needed to confirm these results in other populations. In Queensland, reducing the rate of SGA infants remains a priority (Queensland Government 2018), suggesting that it may be useful to consider targeting underweight mothers in strategies aimed at reducing the proportion of SGA births. Care provided at the preconception stage could focus on dietary advice to ensure healthy maternal weight. High maternal BMI has been previously found to be associated with adverse birthweight outcomes, with overweight and obese women having a higher proportion of LGA babies (Vilcins et al. 2020). LGA neonates are more likely to require intervention during delivery and have higher rates of neonatal intensive care unit admission and resuscitation (Ng et al. 2010). With obesity rates increasing in Australia, there is a need to consider preconception nutrition counselling to assist women in achieving a healthy weight prior to pregnancy.

Diabetes, both gestational and pre-existing, have previously been shown to important predictors of birthweight extremes (Vilcins et al. 2020). Our trend analysis shows a sharp rise in the number of women developing gestational diabetes in this population, with a rise to 13.1% by 2019. These findings contrast with a previous study in the United States which found prevalence of gestational diabetes held steady (7.4 per 100 births in 2005), while prevalence of pre-existing diabetes increased (1.82 per 100 births in 2005) (Lawrence et al. 2008), although it should be noted the study period of this analysis ended five years prior to the sudden rise in gestational diabetes we found from 2010. Of interest, we found that mean birthweight for babies born to women with gestational or pre-existing diabetes did not differ greatly from that of babies born to non-diabetic mothers. However, there was a much higher rate of LGA in these babies. These results are similar to an earlier study in New South Wales, Australia, which found small increases in mean birthweight (23–25 g for boys and girls, respectively), but an 18% increase in LGA births (Hadfield et al. 2009). One potential explanation is medical care practices. In our population, babies born to diabetic mothers had around 2 weeks lower gestational age (data not shown), suggesting that medical care interventions in this group contributed to the mean birthweight being similar but the high rates of LGA. A previous study from the United States showed that women with pre-existing diabetes have a 6-fold increase odds of caesarean section, which rose to an 11-fold risk when women with medical risk factors for caesarean section where excluded (Remsberg et al. 1999). In our trend analysis, babies born to women with gestational diabetes had a similar gestational age to babies whose mothers did not have gestational diabetes, suggesting a different mechanism driving the rates of LGA in this group. Over time, there was an increased proportion of babies born at an appropriate age-standardised weight, suggesting that current care practices are helpful in preventing excessive foetal size in women diagnosed with gestational diabetes. A Cochrane review found that moderate to tight glycaemic control targets, compared with loose targets, was significantly associated with lower rates of LGA (Middleton et al. 2016). Nevertheless, the increasing rates of gestational diabetes suggest a need for increased interventions for maternal health prior to pregnancy.

The trend of decreasing mean birthweight across time, while LGA rates are simultaneously increasing is perplexing, and not previously found in the New South Wales trend analysis (Hadfield et al. 2009). The small decrease in the proportion of SGA births across the study period suggests this is not driven by an increase in smaller babies, when sex and gestational age is accounted for. The most likely explanation is the decline in gestational age across the same period. While the rates of decline differ slightly, the trend lines are broadly similar and may indicate that a declining gestational age is driving the trend to lower birthweight. The decrease in gestational age at first antenatal visit would suggest improved antenatal care across the period in question, which would be expected to increase mean birthweight. Therefore, it is possible these findings indicate that that there are more elective caesareans performed at earlier gestation; however, we are unable to account for this in the current study. Conversely, the trend to higher birthweight across the study period for babies born preterm (< 37 weeks completed gestation) is positive.

Conclusion

Birthweight remains an important predictor of neonatal health and understanding changes in maternal and neonatal patterns can inform care giving practices. This secular trend analysis shows that neonatal and maternal characteristics are changing over time. Birthweight and gestational age decreased, the rate of SGA declined and the rate of LGA changes across the period were non-linear. There was a greater number of births to mothers between 30–39 years of age than previous age years. Rates of gestational diabetes increased over time.