Demography for a closed cohort of pregnancies

Abbreviations

Abbreviation	Definition	Note
ASFR	Age-specific fertility rate
SBR	Stillbirth to live birth ratio
LBWSG	Low birthweight and short gestation
WRA	Women of reproductive age
PLW	Pregnant and lactating women

Overview

This document is intended to represent the demography model for a simulation of a closed cohort of pregnancies. This population structure differs from the typical default Vivarium population structure delineated by age, sex, location, and year only. The closed cohort of pregnancies model for use in the Nutrition optimization pregnancy model will initialize a population that consists of pregnant female simulants only rather than initializing a population of women of reproductive age generally and tracking their pregnancy status over time. Because of this, we must implement a custom age structure of the initialized population to be representative of the pregnant population in any location and year.

GBD Modeling Strategy

Pregnancy and births are not explicit outcomes in the GBD study. However, there are location- and year-specfic GBD covariates related to fertility and births. A more thorough overview can be found on the pregnancy model document

Vivarium Modeling Strategy

While the default population structure of Vivarium simulations is determined according to the age/sex/location/year-specific GBD population count estimates, this custom demography model will be determined instead by the product of the age/sex/location/year-specific GBD population count estimates multiplied by the pregnancy incidence rate for that demographic group.

Relevant parameters are defined below:

Parameters

Parameter	Value	Note
population for use in Vivarium initialization	gbd_population * pregnancy_incidence
pregnancy_incidence	ASFR + ASFR * SBR + incidence_c995 + incidence_c374

Data values

Parameter	Data type	Data ID	Source	Note
gbd_population	population estimate	N/A	get_population: decomp_step=’iterative’
ASFR	Covariate	13	get_covariate_estimates: decomp_step=’iterative’	Assume lognormal distribution of uncertainty
SBR	Covariate	2267	get_covariate_estimates: decomp_step=iterative’	Parameter is not age specific and has no draw-level uncertainty. Use mean_value as location-specific point parameter.
incidence_c995	Incidence rate of abortion and miscarriage cause	c995	como; decomp_step=’step5’	Use the total population incidence rate directly from GBD and do not rescale this parameter to susceptible-population incidence rate using condition prevalence.
incidence_c374	Incidence rate of ectopic pregnancy	c374	como; decomp_step=’step5’	Use the total population incidence rate directly from GBD and do not rescale this parameter to susceptible-population incidence rate using condition prevalence.

Note

AGE SHIFTING: The incidence rates in the table above are measured at the end of pregnancy. However, we will use them to inform the rates of the beginning of pregnancy. This will cause us to initialize a population that is older than it should be by the average duration of pregnancy in the given population. This had a minimal influence in the IV iron simulation, so it was regarded as an acceptable model limitation. However, if this is thought to cause model validation issues, we may revist this strategy and implement an age-shifting adjustment.

Restrictions

Restriction Type	Value	Notes
Male only	False
Female only	True
Age group start	10 to 14 years	ID=7
Age group end	50 to 54 years	ID=15