Exposure to Ambient Particulate Matter during Specific Gestational Periods Produces Adverse Obstetric Consequences in Mice

Abstract

Background: Epidemiological studies associate inhalation of fine-sized particulate matter (PM_2.5) during pregnancy with preterm birth (PTB) and low birth weight (LBW) but disagree over which time frames are most sensitive, or if effects are cumulative.

Objectives: Our objective was to provide experimental plausibility for epidemiological observations by testing the hypothesis that exposure to PM_2.5 during discrete periods of pregnancy results in PTB and LBW.

Methods: For the first study, timed-pregnant B6C3F1 mice were exposed to concentrated ambient PM_2.5 (CAPs) or filtered air (FA) throughout pregnancy [6 h/d from gestational day (GD) 0.5 through GD16.5]. A follow-up study examined the effects of CAPs exposure during discrete gestational periods (1: GD0.5–5.5; 2: GD6.5–14.5; 3: GD14.5–16.5; 4: GD0.5–16.5) aligning to milestones during human development.

Results: In the first experiment, exposure to 160 μg CAPs/m³ throughout pregnancy decreased gestational term by 0.5 d (∼1.1 wk decrease for humans) and birth weight by 11.4% compared with FA. The follow-up experiment investigated timing of CAPs exposure (mean concentrations at 178, 193, 171, and 173 μg/m³ for periods 1–4, respectively). Pregnancy was significantly shortened (vs. FA) by ∼0.4d when exposure occurred during gestational periods 2 and 4, and by ∼0.5d if exposure occurred during period 3. Exposure during periods 1, 2, and 4 reduced birth weight by ∼10% compared with FA, and placental weight was reduced (∼8%) on GD17.5 if exposure occurred only during period 3.

Conclusions: Adverse PM_2.5-induced outcomes such as PTB and LBW are dependent upon the periods of maternal exposure. The results of these experimental studies could contribute significantly to air pollution policy decisions in the future. https://doi.org/10.1289/EHP1029.

Figure 1.

Timeline for inhalation CAPs exposure. Upon arrival, female mice were staged for phase of the estrous cycle. On the third proestrus following two normal cycles, the female was paired with a single male to breed overnight. Upon confirmation of breeding, the female was weighed and assigned to a treatment and to an exposure period. Exposures were 6 h/d, 7 d/wk. Dams were weighed daily before being placed into the exposure box if being exposed or returned to the home cage if not being exposed. Mice from all periods were either euthanized on GD17.5 or allowed to give birth as described in “Methods.” Note: CAPs, concentrated ambient ${\text{PM}}_{2.5}$ (fine-sized particulate matter); NYU, New York University.

Figure 2.

Maternal exposure to inhaled CAPs results in preterm birth and low birth weight. Dams were exposed to CAPs during period 4 (GD0.5–16.5) and were allowed to give birth. Data are from experiment 1 (A, B) and experiment 2 (C, D). In experiment 1, some naïve dams ($n=4$) were used to control for changes resulting from the exposure system. Data for experiment 1 are the $\text{means}\pm \text{standard}\text{error}\left(\text{SE}\right)$ from $n=10$ (FA) or $n=15$ (CAPs); for experiment 2, $n=22$ for each treatment. In all panels, the treatment effect is significant [analysis of variance (ANOVA) $p<0.05$]. Bars in panels A and B with different letters are significantly different based on Fisher’s Least Significant Difference (LSD) post hoc testing. Note: CAPs, concentrated ambient ${\text{PM}}_{2.5}$ (fine-sized particulate matter); FA, filtered air. *$p<0.05$ based on ANOVA.

Figure 3.

Exposure of pregnant mice to CAPs during different exposure periods (experiment 3) is associated with decreased body weight (A), decreased CRL (B), and altered placental weight (C) on GD17.5. The results from analysis of variance (ANOVA) showed significant differences ($p<0.05$) among the groups for each endpoint which was followed by Fisher’s Least Significant Difference (LSD) post hoc testing to determine differences compared with FA. Data are the $\text{means}\pm \text{standard}\text{error}\left(\text{SE}\right)$ from $n=5$ dams from each CAPs exposure period or $n=16$ from the pooled FA control dams. Note: CAPs, concentrated ambient ${\text{PM}}_{2.5}$ (fine-sized particulate matter); CRL, crown-to-rump length; FA, filtered air; GD, gestational day. *$p<0.05$ compared with FA dams based on post hoc testing.

Figure 4.

Maternal exposure to inhaled CAPs during different periods of pregnancy in experiment 3 as described in “Methods” are associated with PTB (A), LBW (B), decreased CRL (C) and decreased SGA (D). The results from analysis of variance (ANOVA) showed significant differences ($p<0.05$) among the groups for each end point; ANOVA was followed by Fisher’s Least Significant Difference (LSD) post hoc testing to determine differences compared with FA. Data are the $\text{means}\pm \text{standard}\text{error}\left(\text{SE}\right)$ from $n=8–11$ dams for CAPs-exposed mice during periods 1 – 4. Because no differences were observed among the four periods for FA control values, the values were pooled ($n=26$). Note: CAPs, concentrated ambient ${\text{PM}}_{2.5}$ (fine-sized particulate matter); CRL, crown-to-rump length; FA, filtered air; LBW, low birth weight; PTB, preterm birth; SGA, size for gestational age. *$p<0.05$ compared with FA dams based on post hoc testing.

Figure 5.

Exposure of pregnant mice does not affect growth rates of offspring (experiment 3). Neonatal body weight gain was computed as a percentage over birth weight (A) or daily body weight gain (percent day-to-day gain) (B). Analysis of percent weight gain compared to birth weight (A) showed no significant differences by ANOVA ($p>0.05$) for the interaction of treatment and time. Comparison of weight gain day-to-day (B) also revealed no significant differences among the groups when data were analyzed by day postpartum. Data are $\text{means}\pm \text{SE}$ from 8–11 dams for each CAPs exposure Period and 26 dams for the pooled FA controls.

Figure 6.

Exposure of pregnant mice to CAPs during different pregnancy periods results in alterations in CRL and AGD in male offspring on PND10 and PND21. CRLs of male offspring were measured on PND10 and PND21 (A, B), and AGDs were measured at these same time points (C, D). The results from analysis of variance (ANOVA) showed significant differences ($p<0.05$) among the groups for each end point; ANOVA was followed by Fisher’s Least Significant Difference (LSD) post hoc testing to determine specific differences among the groups. Data presented are the $\text{means}\pm \text{standard}\text{error}\left(\text{SE}\right)$ from $n=8-11$ dams for each CAPs exposure period and $n=26$ dams for the pooled FA controls. Bars with different letters are significantly different from one another ($p<0.05$). Note: AGD, anogenital distance; CAPs, concentrated ambient ${\text{PM}}_{2.5}$ (fine-sized particulate matter); CRL, crown-to-rump length; FA, filtered air; PND, postnatal day.

Figure 7.

Exposure of pregnant mice to CAPs during different pregnancy periods results in alterations in CRL and AGD in female offspring on PND10 and PND21. CRLs of female offspring were measured on PND10 and PND21 (A, B), and AGDs were measured at these same time points (C, D). The results from analysis of variance (ANOVA) showed significant differences ($p<0.05$) among the groups for each endpoint; ANOVA was followed by Fisher’s Least Significant Difference (LSD) post hoc testing to determine specific differences among the groups. Data presented are the $\text{means}\pm \text{standard}\text{error}\left(\text{SE}\right)$ from $n=8-11$ dams for each CAPs exposure period and $n=26$ dams for the pooled FA controls. Bars with different letters are significantly different from one another ($p<0.05$). Note: AGD, anogenital distance; CAPs, concentrated ambient ${\text{PM}}_{2.5}$ (fine-sized particulate matter); CRL, crown-to-rump length; FA, filtered air; PND, postnatal day.

Figure 8.

Alignment of mouse reproductive timeline to that of humans from the beginning of pregnancy through parturition. This table is based on Theiller stages of mouse development (Theiler 1989) and Carnegie stages of human development (O'Rahilly and Müller 2010).