Embryology · Fetal period, placenta and twins
Abnormal placentation: previa and the accreta spectrum !
- Topic
The placenta can implant too low (previa) or too deep (accreta); both cause serious bleeding in pregnancy or at delivery.
Updated 2026-11-01
See it in 3D: Lower uterine segment · Cervix · Body of uterus · Urinary bladder
What it is
Placenta previa implants in the lower uterine segment and covers or reaches the internal cervical os. In the accreta spectrum the decidua basalis is missing and the villi stick to or invade the myometrium. Both share one central risk factor, a previous caesarean scar, and they often occur together.
Structure
- A normal decidua basalis limits trophoblast invasion; where it is missing, as over a scar, the villi reach the muscle.
- In previa the blastocyst implants near the cervix; part of the placenta lies over the lower segment, which stretches late in pregnancy and makes it bleed.
- Many low-lying placentas in the second trimester move away from the cervix as the lower segment grows («placental migration»).
Previa, accreta and abruption
| Disorder | What goes wrong | Typical sign | Main risk |
|---|---|---|---|
| Placenta previa | Implantation in the lower segment | Painless bleeding | Previous caesarean, multiparity |
| Accreta spectrum | Decidua basalis missing | Placenta will not separate | Previous caesarean with previa |
| Abruption | Premature separation | Painful bleeding, tense uterus | Hypertension, smoking, cocaine |
Clinical notes
- Previa: bleeding can be sudden and recurrent; delivery is usually by caesarean.
- Accreta: massive postpartum haemorrhage when removal of the placenta is attempted; often ends in hysterectomy.
- Shared risk factors: previous caesareans or uterine surgery, curettage, older maternal age, smoking, multiple pregnancy and in vitro fertilization.
In the lab
- Transvaginal ultrasound: locates the placenta and measures its distance to the internal os.
- Ultrasound signs of accreta: placental lacunae, loss of the clear retroplacental zone and thin myometrium; MRI completes the work-up.
Test yourself
Think of the answer, then tap the question to see it.
How is placenta previa told apart clinically from placental abruption?
Previa bleeds painlessly with a soft uterus; abruption bleeds with pain, contractions and a tense uterus.
Which layer is missing in the placenta accreta spectrum?
The decidua basalis, which normally limits trophoblast invasion; without it the villi attach to or invade the myometrium.
Which history carries the highest risk of placenta accreta?
A placenta previa in a woman with one or more previous caesareans: the placenta implants right over the scar.
What is the difference between placenta increta and percreta?
In increta the villi invade the myometrium; in percreta they go through it to the serosa and can reach the bladder.
See it in 3D
The structures of Durgical's 3D anatomy where it is found.
Related
- Placenta: structure, circulation and functions
- Week 1: cleavage, morula, blastocyst and implantation
- Trophoblast and hCG; gestational trophoblastic disease
- Fetal growth and growth restriction
Related conditions
Educational content on embryonic development; not a guide to prenatal diagnosis or to medicines in pregnancy and no substitute for a health professional's advice. No doses are given.
Sources: Placenta previa: MedlinePlus Medical Encyclopedia (MedlinePlus, U.S. National Library of Medicine, 2026); Placental Accreta, Increta and Percreta (March of Dimes, 2022); Langman's Medical Embryology, 13th ed., ch. 8: Third Month to Birth: The Fetus and Placenta (Wolters Kluwer, 2015).
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Representative model, for educational purposes only.