Direct answer: Preeclampsia is most commonly diagnosed after 20 weeks of gestation, but it can develop earlier in rare cases. The typical onset is in the second half of pregnancy (often after 27 weeks) with many cases presenting in the third trimester, while early-onset preeclampsia (before 34 weeks) is less common but recognized. In exceptionally rare instances, features resembling preeclampsia have been reported before 20 weeks, often in association with specific conditions such as molar pregnancy, antiphospholipid syndrome, or other placental disorders. After delivery, preeclampsia can also appear within 48 hours to several weeks postpartum in rare cases. If you or someone you know is pregnant and experiencing new high blood pressure, swelling, headaches, vision changes, upper abdominal pain, or sudden weight gain, seek urgent medical evaluation. These can be warning signs of preeclampsia or related conditions that require monitoring and management by obstetric care providers.