Pregnancy and Menstruation: Why True Periods Don't Occur During Pregnancy

A true menstrual period cannot occur during pregnancy. Menstruation is the shedding of the uterine endometrium when implantation fails. Pregnancy = successful implantation → endometrium is maintained and transformed into decidua.

Bleeding in Pregnancy ≠ Menstruation

• Implantation bleeding: 6–12 DPO. Light spotting, pink/brown, hours–2 days. Not a period.
• Subchorionic hematoma: Blood between chorion and uterine wall. Common (1–3%), often resolves.
• Cervical changes: Ectropion, friability → post-coital spotting, post-exam bleeding.
• Infection: Cervicitis, vaginitis (BV, yeast, trichomonas).
• Subchorionic hemorrhage / threatened miscarriage: Bleeding + cramping, viable fetus.
• Ectopic pregnancy: Bleeding + pain. Emergency.
• Molar pregnancy: Abnormal trophoblast. Vaginal bleeding, high hCG.
• Placental issues (later): Previa, abruption, vasa previa.

Clinical Approach to Bleeding in Pregnancy

1. Quantify: Spotting (pantyliner) vs. bleeding (pad/hour).
2. Assess viability: Transvaginal ultrasound (gestational sac, yolk sac, fetal pole, cardiac activity by ~6 wks).
3. hCG trend: Should double ~48 hrs in early viable pregnancy.
4. Rh status: Rh-negative → RhIg (RhoGAM) if bleeding > spotting or invasive procedure.
5. Counsel: Activity modification (pelvic rest), warning signs (heavy bleeding, severe pain, fever, tissue passage).

Key Message

Any bleeding in pregnancy warrants clinician contact. "Period-like" bleeding is not a period—it's a symptom requiring evaluation. Most early bleeding resolves with healthy pregnancy, but ectopic pregnancy and miscarriage must be excluded.