Classification of Hypertensive Disorders in Pregnancy
The International Society for the Study of Hypertension in Pregnancy (ISSHP) provides the definitive classification system that divides hypertensive disorders into two main temporal categories: hypertension present before or in early pregnancy (before 20 weeks), and hypertension arising de novo at or after 20 weeks' gestation. 1
Primary Classification Framework
The ISSHP classification organizes hypertensive disorders based on timing and etiology:
Hypertension Known Before Pregnancy or Present in First 20 Weeks
Chronic Hypertension is defined as blood pressure ≥140/90 mmHg that either predates pregnancy or develops before 20 weeks' gestation. 2 This category subdivides into:
- Essential hypertension - the most common form, typically accompanied by family history and often associated with overweight or obesity 1
- Secondary hypertension - less common causes including primary renal parenchymal disorders (reflux nephropathy, glomerulonephritis), fibromuscular hyperplasia of renal arteries, or primary hyperaldosteronism 1
White-Coat Hypertension presents as elevated office/clinic BP (≥140/90 mmHg) but normal BP at home or work (<135/85 mmHg). 1, 3 This is not entirely benign and carries increased risk for preeclampsia, affecting up to 25% of patients with elevated clinic readings. 1, 4
Masked Hypertension is characterized by normal clinic BP but elevated readings at other times. 3
Hypertension Arising De Novo at or After 20 Weeks
Transient Gestational Hypertension develops at any gestation but resolves without treatment during pregnancy. 1 Despite its transient nature, this carries approximately 40% risk of subsequently developing gestational hypertension or preeclampsia later in the same pregnancy. 3, 4
Gestational Hypertension is persistent de novo hypertension developing at or after 20 weeks without features of preeclampsia. 1 The risk of complications depends critically on gestational age at onset, with approximately 20-25% progressing to preeclampsia. 4, 5
Preeclampsia is diagnosed when gestational hypertension is accompanied by one or more new-onset conditions including proteinuria (≥0.3 g/day or ≥30 mg/mmol urinary creatinine), renal insufficiency, liver involvement, neurological complications, hematological complications, or uteroplacental dysfunction at or after 20 weeks. 3, 2
Superimposed Preeclampsia occurs when preeclampsia develops on top of pre-existing chronic hypertension, complicating up to 25% of pregnancies with chronic hypertension. 1
Critical Diagnostic Thresholds
Hypertension is uniformly defined as systolic BP ≥140 mmHg and/or diastolic BP ≥90 mmHg. 3, 4
Severe hypertension requires systolic BP ≥160 mmHg and/or diastolic BP ≥110 mmHg. 3, 4
Confirmation Requirements
- For severe hypertension (≥160/110 mmHg): Confirmation must occur within 15 minutes with repeated measurements 3, 4
- For less severe hypertension: Confirmation requires repeated readings over several hours on the same visit or on two consecutive antenatal visits 1, 4
Important Clinical Distinctions
The ISSHP explicitly recommends against using the term "severe preeclampsia" in clinical practice, instead describing preeclampsia as "with or without severe features." 1, 3 This terminology shift reflects more accurate risk stratification.
The 20-Week Gestational Cutoff
The 20-week threshold is critical for classification. 1 However, a common pitfall exists: normal BP first measured after 12 weeks may reflect the physiologic first-trimester BP decrease, potentially masking underlying chronic hypertension. 1 Therefore, prepregnancy or early first-trimester BP documentation is essential for accurate classification.
Ambulatory and Home BP Monitoring
Normal 24-hour ambulatory BP values before 22 weeks should be below:
These thresholds are slightly lower than non-pregnant values. 1 Approximately 25% of automated home devices differ from standard sphygmomanometry, requiring validation against calibrated devices before clinical use. 1
Baseline Testing for Chronic Hypertension
All women with chronic hypertension require baseline testing at first diagnosis to facilitate later detection of superimposed preeclampsia:
- Complete blood count (hemoglobin and platelet count)
- Liver enzymes (AST, ALT, LDH) and function tests (INR, bilirubin, albumin)
- Serum creatinine, electrolytes, and uric acid
- Urinalysis with microscopy and protein-to-creatinine ratio or albumin-to-creatinine ratio
- Renal ultrasound if serum creatinine or urine testing are abnormal 1, 3
Note that while elevated gestation-corrected uric acid associates with worse outcomes, it should not determine delivery timing. 1
Long-Term Implications
Women with any hypertensive disorder of pregnancy face significant long-term cardiovascular risks. 1 Annual medical review is advised lifelong, with goals including achieving prepregnancy weight by 12 months postpartum and maintaining healthy lifestyle through exercise and optimal body weight. 1