Development and validation of a simple algorithm to estimate common gestational age categories using standard administrative birth record data in Ontario, Canada

Figures & data

Table 1. Sex-specific algorithm using three standard birth record fields for the estimation of common gestational age categories: extremely preterm, very preterm, moderate-late preterm and term births.

Criteria	Prematurity flag^a	Weight flag^b	Gestational weight, g^c^,^d
			Male	Female
Extreme preterm (<28 wGA)
1	Extremely preterm	–	–	–
2	No preterm flag recorded	Light for GA	≤763	≤717
3	No preterm flag recorded	Heavy for GA	≤1444	≤1346
4	No preterm flag recorded	Missing	≤1103	≤1042
Very preterm (28–32 wGA)
1	Preterm	Light for GA	≤1444	≤1346
2	Preterm	Heavy for GA	1445–2579	1347–2493
3	Preterm	Missing^e	≤1802	≤1715
4	No preterm flag recorded	Light for GA	764–1443	718–1345
5	No preterm flag recorded	Heavy for GA	1445–2579	1347–2493
6	No preterm flag recorded	Missing	1104–1801	1043–1714
Moderate-late preterm (33–36 wGA)
1	Preterm	Light for GA	1445–2321	1347–2227
2	Preterm	Heavy for GA	≥2580	≥2494
3	Preterm	Missing^e	1803–3664	1716–3542
4	No preterm flag recorded	Light for GA	1445–2321	1347–2227
5	No preterm flag recorded	Heavy for GA	2580–3664	2494–3542
6	No preterm flag recorded	Missing^e	1803–2206	1716–2112
Term (37+ wGA)
1	No preterm flag recorded	Light for GA	≥2322	≥2228
2	No preterm flag recorded	Heavy for GA	≥3665	≥3543
3	No preterm flag recorded	Missing	≥2207	≥2113

wGA: weeks gestational age; GA: gestational age; ICD: International Classification of Diseases.

^a Extreme preterm (short gestation) flag: ICD-10: P07.2; preterm (short gestation) flag: ICD-10: P07.3. All diagnostic fields recorded on the infant’s birth record were considered. ICD-10CA codes were exclusively used during our study period. In Canada, the fifth ICD digit is not recorded; however, fifth digits for P07.2 correspond to births occurring at or before 27 wGA and P07.3 to births occurring 28–36 wGA.

^b Heavy for gestational age (GA) flag: ICD-10: P08^; light for GA flag: ICD-10: P05^. All diagnostic fields recorded on the infant’s birth record were considered. ICD-10CA codes were exclusively used during our study period.

^c Weights exceeding 6000 g were assumed to be misreported in pounds (equivalent to 453.592 g).

^d Sex-specific gestational weight cut-offs from Kramer et al. (2000).

^e Given that the preterm newborn flag encompasses all infants born between 28 and 36 wGA, we ensured that weight cut-offs for children bordering the very preterm (28–32 wGA) and moderate-to-late preterm (33–26 wGA) categories did not overlap by taking the average of the two 50th percentile weight cut-offs reported in the sex-specific growth charts, where necessary; i.e. the highest weight cut-off from the lower gestational age category and the lowest weight cut-off from the higher gestational age category.

Table 2. Agreement between estimated gestational age (GA) category and recorded GA category, based on continuous number of weeks of completed gestational age (wGA) as recorded by medical professional at the time of birth, for births occurring in Ontario, Canada, January 1 2003 through to December 31 2016.

Recorded gestational age	Estimated gestational age N (row %)				Total
	Extreme preterm	Very preterm	Moderate to late preterm	Term
Extreme preterm <28 wGA	6332	614	67	92	7105
	(89.1)	(8.6)	(0.9)	(1.3)
Very preterm 28–32 wGA	404	12,785	5779	444	19,412
	(2.1)	(65.9)	(29.8)	(2.3)
Moderate to late preterm 33–36 wGA	164	5537	99,400	7712	112,813
	(0.2)	(4.9)	(88.1)	(6.8)
Term 37+ wGA	374	550	10,932	1,652,236	1,664,092
	(0.02)	(0.03)	(0.7)	(99.3)
Total	7,274	19,486	116,178	1,660,484	1,803,422

Simple kappa (95% CI): 87.61 (87.49, 87.74); Weighted kappa (95% CI): 89.35 (89.24, 89.46). Number of births missing either recorded or estimated GA and excluded from analysis: 4739 (0.3%).

Table 3. Sensitivity, specificity, positive predictive value (PPV), negative predictive value (NPV) and accuracy, with 95% confidence intervals, for each algorithm category compared to clinical estimate of continuous gestational age (GA).

Estimated GA category	Sensitivity (95% CI)	Specificity (95% CI)	PPV (95% CI)	NPV (95% CI)	Accuracy (95% CI)
Extreme preterm <28 wGA	89.12	99.95	87.05	99.96	99.90 (99.90, 99.91)
	(88.37, 89.84)	(99.94, 99.95)	(86.31, 87.76)	(99.95, 99.96)
Very preterm 28–32 wGA	65.86	99.62	65.61	99.63	99.26 (99.25, 99.27)
	(65.19, 66.53)	(99.62, 99.63)	(65.02, 66.19)	(99.62, 99.64)
Moderate to late preterm 33–36 wGA	88.11	99.01	85.56	99.21	98.33 (98.31, 98.35)
	(87.92, 88.30)	(98.99, 99.02)	(85.37, 85.75)	(99.19, 99.22)
Term 37+ wGA	99.29	94.08	99.50	91.71	98.89 (98.87, 98.90)
	(99.27, 99.30)	(93.95, 94.20)	(99.49, 99.51)	(91.57, 91.84)

Number of births missing either recorded or estimated GA and excluded from analysis: 4739 (0.3%).

Table 4. Sensitivity analysis of agreement between administrative data-based algorithm and recorded gestational age categories, according to specific infant characteristics.

Birth characteristic	Simple kappa (95% CI)	Weighted kappa (95% CI)	Effective sample size
Multiple birth set
Singleton birth	87.41 (87.26, 87.55)	88.95 (88.82, 89.09)	1,744,582
Multiple birth	80.85 (80.43, 81.26)	84.47 (84.12, 84.82)	58,840
Birth sex
Female	87.46 (87.27, 87.65)	89.16 (88.98, 89.33)	877,926
Male	87.74 (87.57, 87.91)	89.51 (89.36, 89.66)	925,496
Birth year
2003	84.19 (83.63, 84.74)	86.87 (86.39, 87.36)	124,099
2004	84.71 (84.18, 85.25)	86.96 (86.48, 87.44)	127,001
2005	85.08 (84.56, 85.61)	87.31 (86.84, 87.78)	127,970
2006	85.54 (85.03, 86.04)	87.48 (87.02, 87.94)	129,630
2007	86.53 (86.04, 87.02)	88.39 (87.95, 88.83)	131,902
2008	87.08 (86.60, 87.55)	88.83 (88.40, 89.26)	133,046
2009	87.69 (87.21, 88.17)	89.52 (89.09, 89.94)	123,301
2010	87.62 (87.15, 88.09)	89.34 (88.91, 89.76)	130,778
2011	89.08 (88.64, 89.52)	90.55 (90.15, 90.94)	130,408
2012	89.31 (88.87, 89.74)	90.65 (90.25, 91.05)	131,459
2013	89.27 (88.83, 89.71)	90.80 (90.41, 91.19)	128,977
2014	89.72 (89.28, 90.15)	91.05 (90.65, 91.45)	128,645
2015	90.00 (89.57, 90.43)	91.35 (90.97, 91.74)	127,785
2016	90.15 (89.73, 90.57)	91.52 (91.14, 91.89)	128,421
Birthweight
<1500 g	67.05 (66.02, 68.08)	63.27 (62.24, 64.30)	18,040
1500–2499 g	67.05 (66.61, 67.49)	69.84 (69.45, 70.23)	96,491
2500–2999 g	91.49 (91.28, 91.70)	91.23 (91.01, 91.45)	293,705
3000–3499 g	86.88 (86.48, 87.28)	86.08 (85.65, 86.51)	665,993
3500–3999 g	71.56 (69.83, 73.29)	69.15 (67.37, 70.93)	532,517
4000–4499 g	67.72 (63.65, 71.79)	63.32 (59.18, 67.46)	169,972
4500+ g	74.21 (67.95, 80.47)	74.61 (68.57, 80.65)	31,198

Kramer MS, Platt RW, Wen SW, Joseph KS, Allen A, Abrahamowicz M, et al. 2000. A new and improved population-based Canadian reference for birth weight for gestational age. Pediatrics 108:E35.

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