At what gestational age (GA) does preeclampsia typically start?

When Does Preeclampsia Start in Pregnancy?

Preeclampsia typically develops after 20 weeks of gestation, defined by the de novo appearance of hypertension accompanied by proteinuria or other signs of organ dysfunction. 1

Definition and Timing of Onset

Preeclampsia is a pregnancy-specific syndrome characterized by:

• New onset hypertension (≥140/90 mmHg) after 20 weeks gestation
• Accompanied by significant proteinuria (≥0.3 g/24h) or other maternal organ dysfunction 1, 2

The timing of onset is significant:

• By definition, preeclampsia cannot be diagnosed before 20 weeks gestation 1
• It occurs after mid-gestation (post 20 weeks) 1
• It can develop at any point from 20 weeks until delivery and even postpartum 3

Classification Based on Timing

Preeclampsia is classified as:

• Preterm preeclampsia (delivery <37 weeks)
• Term preeclampsia (delivery ≥37 weeks)
• Postpartum preeclampsia 3

Pathophysiology and Early Changes

While clinical signs appear after 20 weeks, the initial pathogenic mechanisms begin much earlier:

• The cytotrophoblast fails to properly remodel spiral arteries in early pregnancy
• This leads to placental hypoperfusion and ischemia
• The ischemic placenta releases factors causing generalized maternal endothelial dysfunction 2

Monitoring and Risk Assessment

For women at risk of developing preeclampsia:

• Before 32 weeks: monitoring at least every 3 weeks
• After 32 weeks: monitoring at least every 2 weeks until delivery 1

Women with risk factors (such as first pregnancy, previous preeclampsia, maternal age ≥40, BMI ≥35, family history) require closer monitoring 1.

Atypical Presentations

While rare, preeclampsia can occur before 20 weeks in specific situations:

• Associated with trophoblastic diseases
• Associated with antiphospholipid syndrome
• In cases of fetal triploidy or other chromosomal abnormalities 4, 5

However, these are exceptional cases and not the typical presentation of preeclampsia.

Clinical Implications

The onset after 20 weeks has important clinical implications:

• Monitoring for signs and symptoms should intensify after this gestational age
• Any new hypertension or proteinuria after 20 weeks should prompt evaluation for preeclampsia
• Women should be educated about symptoms to report (headache, visual disturbances, epigastric pain) 1

Common Pitfalls

1. Failing to recognize that physiological BP decreases in the first trimester may mask pre-existing hypertension, leading to misdiagnosis of later hypertension as new-onset 1

2. Not considering that preeclampsia can develop postpartum, with cases reported up to 12 weeks after delivery 6, 3

3. Overlooking atypical presentations before 20 weeks in special circumstances, which though rare, can occur 4, 5