Accuracy of self-reported birth outcomes relative to birth certificate data in an Internet-based prospective cohort study

Abstract

Background:

The accuracy of birth outcome data provided by Internet-based cohort study participants has not been well studied.

Methods:

We compared self-reported data on birth characteristics in Pregnancy Study Online (PRESTO), an Internet-based prospective cohort study of North American pregnancy planners, with birth certificate data. At enrollment, participants were aged 21–45 years, attempting conception, and not using fertility treatment. Women completed online questionnaires during preconception, early and late pregnancy, and postpartum. We requested birth certificate data during 2014–2019 from seven health departments in states with the most participants. After restricting to singleton births, we assessed specificity, sensitivity, and agreement comparing self-reported data from postpartum questionnaires with birth certificate data for gestational age at delivery (GA) and birthweight (grams). Our primary measure of self-reported GA (weeks) was calculated as: [280-(due date-birth date)]/7. We used log-binomial regression to assess predictors of agreement.

Results:

We linked 85% (771/909) of women in selected states. Median age of women was 30 years (range: 21–42), 84% had ≥16 years of education, nearly 96% were married, 12% had household incomes <$50,000, 32% were parous, and 85% identified as non-Hispanic White. Median recall interval was 6 months. Among those with self-reported data, 89% reported the same GA as the birth certificate and 98% reported GA within 1 week of the birth certificate. Self-report of preterm birth (GA<37 weeks) agreed with information from birth certificates for 100% of women; sensitivity was 100% and specificity was 99%. Self-reported low birthweight (<2,500 grams) agreed with birth certificates for 93% of women; sensitivity and specificity were 92.5% and 99.7%, respectively. Predictors of poorer agreement included higher parity and longer pregnancy attempt time for GA, and lower education and longer recall interval for birthweight.

Conclusion:

Self-reported data on GA and birthweight from an Internet-based cohort showed high accuracy compared with birth certificates.

Background

Despite the increasing popularity of Internet-based studies,^1–5 there has been limited investigation of the accuracy of self-reported data provided by Internet-based study participants.⁶ Online questionnaires have several validity-related advantages over paper-based questionnaires, including built-in internal consistency checks, pop-ups for inadvertently skipped questions, and lower missingness.^7,8 Internet-based study participants may also feel more comfortable sharing personal information online relative to face-to-face interviews.⁸ However, without special efforts, verifying the identities of Internet-based study participants is challenging and reporting accuracy may be poorer for some variables (e.g., birth dates) due to confidentiality concerns.

We assessed the accuracy of self-reported data on birth characteristics in a prospective cohort study that recruits and follows participants via the Internet.

Methods

Study design

Pregnancy Study Online (PRESTO) is an Internet-based prospective cohort study of U.S. and Canadian residents.⁹ Recruitment began in June 2013; enrollment and follow-up are ongoing. Eligible women are aged 21–45 years, trying to conceive, and not using contraception or fertility treatments.⁹

On the baseline questionnaire, participants report data on socio-demographics, anthropometrics, behaviors, menstrual cycle length and regularity (i.e., being able to “predict about when the next period will start”), and reproductive and medical history. On bimonthly follow-up questionnaires, participants update the date of their last menstrual period (LMP) and pregnancy status. Pregnant women complete an early pregnancy questionnaire (median gestational weeks at completion=8), which ascertains pregnancy confirmation method and due date. The late pregnancy questionnaire, completed at approximately 32 weeks’ gestation, updates data on pregnancy status and due date. In October 2017, we added a postpartum questionnaire (PPQ) to ascertain complications of pregnancy, birth outcomes, and maternal and infant health. Participants are invited to complete the PPQ six months after their reported due date. We invited all former participants who delivered before March 2017 (29% of PPQ respondents) to complete an abbreviated PPQ. In this subgroup, median time since delivery was 30 months, with most variability due to late introduction of the PPQ (Table 1).

Table 1.

Baseline characteristics of 1,107 participants by PPQ response, PRESTO (2013–2019)^a

Characteristic	Completed PPQ	Did not complete PPQ
Number of women	909	198
Recall interval, months (median, range)b	6 (0–66)	--
Retrospectively-invited women	30 (8–66)	--
Prospectively-invited women	6 (0–39)	--
Age at baseline, years (median, range)	30 (21–42)	29 (21–42)
Education, years (%)
≤12	1.5	4.5
13–15	11.1	26.8
16	31.2	32.8
≥17	56.2	35.9
Household income, U.S. dollars (%)
<50,000	10.0	20.2
50,000–99,999	34.1	36.9
100,000–149,999	30.0	24.7
≥150,000	23.6	13.6
Refused/missing	2.3	4.6
White, non-Hispanic (%)	85.8	83.3
Body mass index, kg/m2 (median, range)c	24.2 (16.7–57.9)	25.1 (15.9–51.3)
Current regular smoker (%)	2.4	6.1
Parity
1 previous birth (%)	22.2	28.8
≥2 previous births (%)	6.9	14.1
History of infertility (%)d	9.1	8.1
Menstrual cycle irregularity (%)	11.9	15.7
Time to index pregnancy, cycles (median, range)	4 (1–105)	5 (1–44)

PPQ=postpartum questionnaire.

^aRestricted to states for which we performed linkage (MA, CA, MI, TX, PA, OH, FL).

^bPPQ completion date - infant’s date of birth. The PPQ was launched in November 2017 and was sent to women at 6 months’ postpartum. Women invited retrospectively to complete their PPQ (N=226) delivered infants before March 2017; all other women (N=683) were invited prospectively to complete their PPQ survey at 6 months postpartum. Late pregnancy questionnaire respondents who had already delivered their infants were transitioned to the abbreviated PPQ.

^cWeight (kg)/height (m)²

^dCalculated among 651 women who reported having tried to conceive previously.

Both PPQ versions included identical questions on estimated infant due date, infant birth date, GA at delivery (“how many weeks pregnant were you when you gave birth?” in “weeks” and “days”), and birthweight (“What was your baby’s birthweight?” in ”pounds” and ”ounces”). For consistency with birth certificate data, we converted birthweight to grams as follows: [(pounds*16) + ounces)]*28.3495.

Acquisition of birth certificate data

PRESTO links questionnaire data with birth certificate data from state registries with the largest number of participants. Data were available from Massachusetts (MA), Michigan (MI), Texas (TX), Pennsylvania (PA), Florida (FL), and Ohio (OH) in 2014 through 2019; and from California (CA) in 2014 through 2018. Supplemental Table 1 describes the state-specific linkage procedures.

We assessed accuracy and agreement between questionnaire (PPQ) vs. birth certificate data for completed GA at delivery (weeks) and birthweight among singleton births. We computed self-reported completed weeks’ gestation three different ways (rounded down to nearest gestational week):

1. [280 days - (infant due date - birth date)]/7

2. [infant birth date - LMP]/7

3. completed weeks and days of gestation (“directly-reported” GA).

We considered “difference in infant due date and birth date” (method #1) the most accurate GA measurement because the due date assigned by the clinician reflects an integrated assessment that generally includes early ultrasound and LMP, with possible revision as the pregnancy evolves.^10–12

Treating the birth certificate as the gold standard, we calculated sensitivity and specificity¹³ and intraclass correlation coefficients (ICCs). We calculated Kappa statistics¹⁴ to evaluate agreement between categorical variables. We used modified Poisson regression to estimate probability ratios (PR) and 95% confidence intervals (CI) for predictors of agreement, selected based on previously-examined variables in the literature: recall interval, age, education, parity, menstrual cycle length, menstrual irregularity, infertility history, and time-to-pregnancy.

Results

On the PPQ, 2,682 women reported having delivered a singleton birth during January 1, 2014 through December 31, 2019 (Supplemental Figure 1). Of those residing in states with registry linkage (N=909), 771 (85%) were successfully linked. Matching ranged from 58.1% (FL) to 99.1% (MI) (Supplemental Table 1).

Median age of women residing in registry-linked states was 30 years (range: 21–42). Nearly 96% were married, 84% had a college degree, 12% had household incomes <$50,000, 85% identified as non-Hispanic White, and 32% were parous. Median recall interval (i.e., PPQ completion date-infant birth date) was 6 months (range: 0–66 months). PPQ respondents had greater education and income, lower parity, and lower BMI at enrollment than non-respondents, and were more likely to be non-smokers and identify as non-Hispanic White (Table 1).

Among the women residing in states where we obtained complete registry data (Supplemental Table 2), PPQ respondents were less likely than non-respondents to have low birthweight infants and deliver preterm, and were more likely to have macrosomic infants. Little difference in response was observed for very low birthweight.

Among the women with complete GA data from birth certificates and the PPQ (Supplemental Figure 1), 98% reported GA within 1 week of the birth certificate (Figure 1a). A slightly higher percentage overerestimated (6%) than underestimated (5%) their GA. Self-reported preterm birth agreed with birth certificates for 100%; sensitivity and specificity for preterm birth were 100% (95% CI: 92.7–100.0%) and 99.4% (95% CI: 98.6–99.8%), respectively. When GA was categorized as <34, 34–36, 37–38, 39–41, and ≥42 weeks, self-reported GA agreed with birth certificates for 100%, 100%, 87.5%, 98.3%, and 66.7% of women, respectively (Supplemental Table 3). Agreement was high for birth certificate vs. self-reported GA estimated using birth and due dates (weighted Kappa=0.92, 95% CI: 0.90–0.95). Agreement was slightly lower for directly-reported GA (weighted Kappa=0.89, 95% CI: 0.85–0.92), and appreciably lower for GA estimated as the difference between birth date and LMP (weighted Kappa=0.76, 95% CI: 0.71–0.80). The ICC for birth certificate vs. self-reported GA estimated using birth and due dates was 0.97 (95% CI: 0.96–0.97). ICCs were lower for directly-reported GA (0.91, 95% CI: 0.90–0.93) and LMP-estimated GA (0.69, 95% CI: 0.65–0.72).

Figure 1.

Comparison of birth certificate and self-reported data from the postpartum questionnaire, PRESTO (2013–2019).

Of the PPQ respondents who were successfully linked and had complete birthweight data (Supplemental Figure 1), 94% reported birthweight within 100 grams of the birth certificate, with roughly equal percentages underreporting and overreporting birthweight (Figure 1b). Self-reported low birthweight agreed with birth certificates for 93% of women (Supplemental Table 4). Sensitivity and specificity of self-reported low birthweight relative to the birth certificate were 92.5% (95% CI: 80.1–97.4%) and 99.7% (95% CI: 99.0–99.9%), respectively. Agreement between self-reported categorical birthweight and birth certificate data was high (weighted Kappa=0.94, 95% CI: 0.93–0.96). The ICC for birth certificate vs. self-reported birthweight was 0.97 (95% CI: 0.97–0.98).

After controlling for all other potential predictors (Table 2), exact agreement in GA was lower among women with higher parity (≥2 previous births vs. none: PR=0.86, 95% CI: 0.74–1.00) and longer time-to-pregnancy (≥12 vs. <6 cycles: PR=0.84, 95% CI: 0.71–0.99). Agreement in birthweight (within 100 g) was lower among women with lower education (<16 vs. ≥17 years: PR=0.93, 95% CI: 0.87–1.01) and longer recall interval (≥36 vs. <12 months: PR=0.83, 95% CI: 0.70–0.99).

Table 2.

Predictors of agreement in gestational week and birthweight comparing birth certificate and self-report, PRESTO (2013–2019)

		Exact agreement of gestational week		Birthweight within 100g
		Unadjusted	Adjusteda	Unadjusted	Adjusteda
Predictor	N	PR (95% CI)	PR (95% CI)	PR (95% CI)	PR (95% CI)
Parity, previous births
0	530	Reference	Reference	Reference	Reference
1	185	0.97 (0.91, 1.03)	0.97 (0.91, 1.04)	0.99 (0.95, 1.04)	0.98 (0.94, 1.03)
≥2	56	0.87 (0.75, 1.00)	0.86 (0.74, 1.00)	1.05 (1.00, 1.09)	1.06 (1.01, 1.11)
Age, years
<25	33	Reference	Reference	Reference	Reference
25–29	283	1.13 (0.94, 1.36)	1.14 (0.95, 1.36)	1.05 (0.94, 1.17)	1.02 (0.91, 1.14)
30–34	356	1.14 (0.95, 1.37)	1.15 (0.96, 1.37)	1.03 (0.92, 1.15)	0.99 (0.89, 1.11)
≥35	99	1.10 (0.90, 1.33)	1.12 (0.93, 1.36)	1.04 (0.93, 1.17)	1.02 (0.90, 1.15)
Recall interval, months
<12	557	Reference	Reference	Reference	Reference
12–23	77	1.03 (0.96, 1.10)	1.03 (0.96, 1.11)	0.95 (0.88, 1.02)	0.94 (0.87, 1.01)
24–35	103	0.91 (0.82, 1.00)	0.91 (0.83, 1.00)	0.94 (0.88, 1.00)	0.94 (0.88, 1.00)
≥36	34	0.98 (0.87, 1.12)	0.97 (0.86, 1.10)	0.83 (0.70, 0.98)	0.83 (0.70, 0.99)
Education, years
<16	101	0.98 (0.90, 1.06)	1.02 (0.94, 1.11)	0.94 (0.87, 1.01)	0.93 (0.87, 1.01)
16	232	0.96 (0.90, 1.02)	0.97 (0.91, 1.03)	0.99 (0.96, 1.03)	0.99 (0.95, 1.03)
≥17	438	Reference	Reference	Reference	Reference
Time to index pregnancy, cycles
<6	470	Reference	Reference	Reference	Reference
6–11	248	1.03 (0.98, 1.08)	1.02 (0.97, 1.07)	0.98 (0.94, 1.02)	0.98 (0.95, 1.02)
≥12	53	0.83 (0.70, 0.97)	0.84 (0.71, 0.99)	0.95 (0.87, 1.04)	0.97 (0.89, 1.06)
Menstrual cycle regularity
Regular	400	Reference	Reference	Reference	Reference
Irregular	90	1.01 (0.93, 1.10)	1.04 (0.96, 1.13)	1.02 (0.97, 1.07)	1.01 (0.96, 1.07)
Used hormones most of time	281	1.04 (0.98, 1.09)	1.02 (0.96, 1.08)	1.00 (0.96, 1.04)	0.99 (0.95, 1.03)
Menstrual cycle length, days
<26	47	0.92 (0.81, 1.05)	0.93 (0.81, 1.06)	1.03 (0.96, 1.09)	1.01 (0.95, 1.08)
26–31	576	Reference	Reference	Reference	Reference
≥32	148	0.96 (0.90, 1.03)	0.94 (0.88, 1.01)	1.03 (0.99, 1.07)	1.02 (0.98, 1.06)

PR = probability ratio. All variables were ascertained at baseline (preconceptionally) with the exception of recall interval and time to index pregnancy. Column numbers of participants represent those with complete data on either gestational age or birthweight.

^aAdjusted for all other variables in table.

Predictors of agreement in directly-reported GA were similar to those based on self-reported due and birth dates (Supplemental Table 5). One exception was that younger age (<25 years) was more strongly associated with poorer agreement. When we assessed predictors of agreement in LMP-estimated GA, agreement was slightly lower among women with higher parity, longer recall interval, and irregular cycles, and appreciably lower among women with longer menstrual cycle lengths.

Comment

In this Internet-based cohort study, mothers reported infant birthweight and GA with high accuracy compared with birth certificate data. Accuracy for GA was highest when using self-reported due date (i.e., ‟280-(due date-birth date)]/7”), which is typically the due date assigned by the clinician, incorporating ultrasound evidence during gestation. Women are also more likely to remember their due date than gestational weeks at delivery. As expected, accuracy was slightly lower for directly-reported GA, and appreciably lower for LMP-estimated GA.

Agreement in GA was lower among women with higher parity and longer time-to-pregnancy, while agreement in birthweight was lower among those with lower education and longer recall intervals. Findings were similar across other definitions of self-reported GA, with the exception of LMP-estimated GA, for which longer menstrual cycle length (≥32 days) was associated with considerably poorer agreement. As the menstrual cycle deviates from 28 days, we expect greater misclassification of the LMP-based estimate.

Our findings agree with previous studies of maternal recall based on interviews^15–20 or paper-based questionnaires.^21–27 Most studies show relatively high agreement for GA and birthweight (Kappa or ICCs>0.85), up to 30 years after delivery.²⁴ The sole Internet-based study, Pregnancy and Infant DEvelopment (PRIDE), evaluated accuracy of perinatal data collected via online questionnaires by 882 pregnant Dutch women aged ≥18 enrolled between 8–12 weeks’ gestation.⁶ Self-administered questionnaires completed by mothers two months after the estimated delivery date were compared with obstetric records. Correlations were high for GA (ICC=0.91) and birthweight (ICC=0.96). Sensitivity and specificity were 90% and ≥99% for post-term birth, 98% and ≥99% for preterm birth, and 100% and ≥99% for low birthweight. There were no consistent predictors of agreement, though, contrary to PRESTO, lower gravidity and parity were associated with reduced accuracy in reported GA (calculated from infant due dates and birth dates). Differences between PRESTO and PRIDE were timing of enrollment (preconception vs. first trimester of pregnancy) and median recall interval (six vs. two months).

Though PRESTO treated birth certificate data as the gold standard, such data are subject to error. A study comparing birthweight on birth certificates with hospital records showed only 92% exact agreement between the data sources.²⁸ Other limitations include small numbers of low birthweight and non-term infants, lack of evaluation of birth outcomes with lower reporting accuracy (e.g., head circumference), and shorter recall interval (median=6 months, longest interval=5 years). PRESTO respondents differed from the U.S. general population in terms of socioeconomic status (e.g., higher education and income)²⁹ and behaviors (e.g., lower smoking prevalence),³⁰ which may limit generalizability of results to other populations and potentially explain the weaker associations observed for established predictors of recall (e.g., low maternal education).

The present report demonstrated high accuracy of self-reported GA and infant birthweight among Internet-based cohort study participants recruited preconceptionally and followed through the postpartum period. Findings may inform interpretation of self-reported data on birth characteristics from Internet-based cohort studies with similar participant demographics and recall intervals.