Two kids are better than none (or one, or three or more)

[Ivor the Engineer]

http://www.ahjonline.com/article/S0002-8703(09)00897-7/abstract

Background

Prior studies relating parity with maternal cardiovascular disease (CVD) have been performed in relatively small study samples without accounting for pregnancy-related complications associated with CVD.


Methods

We examined the associations between parity and maternal risk of later-life CVD in a population-based cohort study using data from the Swedish population registers. Women born from 1932 to 1955 were followed until the occurrence of CVD, death, emigration, or end of follow-up (December 31, 2005). Cox proportional hazards models were used to estimate associations between parity and risk of CVD accounting for birth year, yearly income, education level, country of birth, hypertension (pregestational hypertension or gestational hypertension, with or without proteinuria), diabetes (type 1, type 2, or gestational diabetes), preterm birth, small for gestational age, and stillbirth.


Results

During a median follow-up time of 9.5 years (range 0-23.5), there were 65,204 CVD events in the full sample of women. Among 1,332,062 women, parity was associated with CVD in a J-shaped fashion, with 2 births representing the nadir of risk (global P value < .0001). Upon accounting for pregnancy-related complications in a subset of women with at least 1 childbirth after 1973 (n = 590,725), the association of parity with CVD was similar. Compared with women with 2 childbirths, the multivariable-adjusted hazard ratios (95% CIs) for women with 1 and ≥5 births were 1.09 (1.03-1.15) and 1.47 (1.37-1.57), respectively.


Conclusions

In conclusion, parity was associated with incident maternal CVD in a J-shaped fashion, even after accounting for socioeconomic factors and pregnancy-related complications.

The authors postulate why the results of their study indicate that women who have more or less than two pregnancies have a higher risk of developing heart disease:

Possible mechanisms

Epidemiologic studies have demonstrated that increasing number of pregnancies is associated with maternal dyslipidemia,17 adiposity,18 and type 2 diabetes.19 Two prior studies that accounted for these CVD risk factors still demonstrated a significant positive association between parity and CVD.4, 5 Normal pregnancy is associated with physiologic changes that alter mediators in various cardiovascular pathways. For example, during pregnancy, there is up-regulation of the renin-angiotensin-aldosterone system20; increased peripheral insulin resistance21; lipid changes including increased free fatty acids, triglycerides, and cholesterol22; and alterations in immune modulation, hemostasis, and endothelial function.23 It is possible that these alterations may result in cumulative derangements over successive pregnancies that eventually lead to increased CVD risk.

Nulliparity conferred a modest risk of maternal CVD in our analyses. It is unclear what proportion of nulliparous women had a history of involuntary infertility and what proportion had made an active decision not to have children. Assuming that there was a relatively high incidence of infertility among nulliparous women in our population, it is possible that conditions such as polycystic ovarian syndrome, which itself is related to cardiometabolic risk factors,24 may have contributed to our findings. Thyroid hormone disorders, which have increased prevalence among women experiencing infertility,25 have also been modestly linked to incident CVD26 and could partly mediate the excess risk associated with nulliparity. The excess risk for women having 1 compared with 2 pregnancies may also reflect involuntary infertility among women having only 1 term pregnancy.

As this is a population-based study, it is not possible to tease out details about which mechanisms connect parity and specific CVD outcomes. Rather, it should be noted that total CVD, as well as the more specific stroke and heart failure end points, comprises a whole range of different pathophysiologies. It is likely that no matter what the mechanisms by which parity is associated with different types of CVD, those mechanisms will be operative in only some of the pathophysiologic processes that can cause disease.

 

[blutoski]

Did they say what the risk levels actually were for 0,1,2,3,4,5 kids? ie: is the benefit of choosing 2 vs 1 or 3 clinically significant? (as opposed to statistically significant?)

 

[Tiktaalik]

The authors postulate why the results of their study indicate that women who have more or less than two pregnancies have a higher risk of developing heart disease:

Actually, it appears that it says two childbirths, not two pregnancies.

 

[casebro]

Hmmm, do they say anything about correcting for age at delivery?

Five kids means the last one was probably 10 years later than the usual first ones? I'm comparing my family of five kids, the family two doors down with 6 kids, the family across the street with seven. All very close to two year cycles. First kids, moms were early 20s. Last kids, moms were 35 years old. Possible interpretation: Mid 20s is the best time for moms to have kids.