Supplemental Table.

Definitions of study outcomes and their coding in Hospital Episode Statistics

Outcome	Numerator/coding	Denominator/coding
Using maternal data:
Stillbirth (fetal death)	Defined using ICD-10 code (Z37.1) or birth status field (birstat_1=2,3,4) in maternity tail for providers with over 95% data completeness.	All singleton births
	In the United Kingdom stillbirth is defined as birth without signs of life occurring at or after 24+0 completed gestational weeks, based on estimated due date calculated using universally offered ultrasound scan at 11–13 weeks’ gestation.	This dataset does not contain sufficient information to distinguish between antepartum and intrapartum stillbirth; in England in 2018 (the latest date for which this information is available), 9 in every 10 stillbirths were antepartum.21
Preterm birth	Defined using gestational age field in HES maternity tail (gestat_1<37)	All singleton births, excluding records missing information on gestational age
Small for gestational age	Defined as less than the tenth birthweight centile using the UK-WHO charts.19 Birthweight centiles are calculated using birthweight (birweit_1), gestational age (gestat_1), sex of baby (sexbaby_1) fields in maternity tail	All singleton births, excluding records missing information on gestational age, birthweight or sex of baby
Preeclampsia or eclampsia	Defined using the ICD-10 codes O14 (preeclampsia) and O15 (eclampsia)	All singleton births
Induction of labor	Defined using the delivery onset field (delonset=3,4,5) from the maternity tail. Failed induction (ICD-10 code O61) is also included in the numerator as this represents intention to treat.	All singleton births, excluding elective cesarean delivery; and records missing information on delivery onset
Elective cesarean delivery (ELC)	ELC is defined using OPCS code R17	All singleton births
Emergency cesarean delivery (EMCS)	EMCS is defined using OPCS codes R18/R25.1	All singleton births
Instrumental delivery	Instrumental birth is defined using OPCS codes R21/R22	All singleton births
Unassisted delivery	Unassisted birth is defined using OPCS code R23/R24	All singleton births
Maternal length of stay after birth (3 or more d)	Length of stay is defined as the number of days between date of discharge and date of admission for the birth episode.	All singleton births with nonmissing date of discharge information and date of delivery before January 28, 2021 (to allow for 3-d follow-up)
Maternal readmission (42 d)	Maternal readmission is defined as unplanned, overnight readmission to hospital within 42 d of giving birth, excluding those accompanying an unwell baby. Mothers readmitted with the following admission method codes: 21, 22, 23, 24, 28, 2A, 2B, 2D, 31, 32, 82, 83 within 42 days of birth.	All singleton births with nonmissing date of discharge information and date of delivery before December 19, 2020 (to allow for 6-wk follow-up). Women who died before discharge or were not discharged within 42 d of delivery were excluded.
Using maternal-neonatal linked data:
Neonatal specialist care	Neonatal specialist care is defined using the “neocare” variable in HES, and includes values 1=Special care: care given in a special nursery, transitional care ward or postnatal ward, which provides care and treatment exceeding normal routine care; 2 = Level 2 intensive care (high dependency intensive care); and 3 = Level 1 intensive care (maximal intensive care)	All singleton, term births with nonmissing information on neonatal specialist care
Neonatal adverse outcome indicator (E-NAOI)	E-NAOI is defined as births with any of the following outcomes: birthweight<1500 g, gestational age under 32 completed weeks, neonatal death within 28 d, respiratory distress syndrome, seizure, intraventricular hemorrhage (grade 3 or 4), cerebral infarction, periventricular leukomalacia, birth trauma (intracranial hemorrhage paralysis owing to brachial plexus injury, skull or long bone fracture), hypoxic ischemic encephalopathy, necrotizing enterocolitis, sepsis/septicemia, pneumonia, respiratory disease (respiratory failure, primary atelectasis, chronic respiratory disease originating in the perinatal period, bacterial meningitis, resuscitation (intubation/chest compression), mechanical ventilation/CPAP/high flow nasal oxygen, central venous or arterial catheter, pneumothorax requiring intercostal catheter, any intravenous fluids, any body cavity surgical procedure, therapeutic hypothermia. Coding of these diagnoses and procedures can be found in Knight et al,20 2018, Supplemental Table.	All liveborn singleton term births with nonmissing information on gestational age and birthweight
Neonatal length of stay after birth (3 or more d)	Length of stay is defined as the number of days between date of discharge and date of admission for the birth episode.	All singleton births with nonmissing date of discharge information and date of birth before January 28, 2021 (to allow for 3-d follow-up)
Neonatal readmission (28 d)	Neonatal readmission is defined as unplanned, overnight readmission to hospital within 28 d of birth, excluding those accompanying an unwell mother. Babies readmitted with the following admission method codes: 21, 22, 23, 24, 28, 2A, 2B, 2D, 31, 32, 82, 83 within 28 days of birth.	All singleton neonates with nonmissing date of discharge information and date of birth before January 3, 2021 (to allow for four-week follow-up). Babies who died before discharge or were not discharged within 28 d of birth were excluded.

CPAP, continuous positive airway pressure therapy; ICD-10, International Classification of Diseases, tenth revision; OPCS, Censuses and Surveys Classification of Surgical Operations and Procedures; UK-WHO, United Kingdom–World Health Organization.

Gurol-Urganci et al. Maternal and perinatal outcomes of pregnant women with SARS-CoV-2. Am J Obstet Gynecol 2021.