Placenta Previa
			The placental edge covers the internal cervical os. Follow-up ultrasonography is recommended at 32 weeks of gestation.
Low lying placenta
	The placental edge is less than 2 cm from the internal os but not covering the internal os. Follow-up ultrasonography is recommended at 32 weeks of gestation [4]
LifeART  images © 2006 Lippincott Williams & Wilkins. All rights reserved.

In 2014 the Executive Summary of a Joint Eunice Kennedy Shriver National Institute of Child Health and Human Development, Society for Maternal-Fetal Medicine, American Institute of Ultrasound in Medicine, American College of Obstetricians and Gynecologists, American College of Radiology, Society for Pediatric Radiology, and Society of Radiologists in Ultrasound Fetal Imaging Workshop recommended eliminating the terms partial and marginal and only retaining the terms placenta previa and low-lying placenta.
 

Placenta previa occurs in one in 200 to 250 births overall, but is much more common if a woman has given birth before, has had a cesarean section, has had placenta previa with a previous pregnancy, or is over the age of 35. It is uncommon in nulliparous women (women who have never given birth) [5].

The main symptom of placenta previa is vaginal bleeding. The bleeding is typically painless unless there is coexisting abruption or labor. About 20% of third trimester bleeding may be attributed to placenta previa[ 5]. Nearly all cases of placenta previa are delivered by cesarean section. Infrequently patients with marginal previa and minimal bleeding are allowed to deliver vaginally as are patients with intrauterine fetal demise (stillbirth)  or a previable pregnancy[6].

REFERENCES
1.Becker RH et al., ,The relevance of placental location at 20-23 gestational weeks for prediction of placenta previa at delivery: evaluation of 8650 cases. Ultrasound Obstet Gynecol. 2001 ;17(6):496-501.
PMID: 11422970
2.Mustafá SA, et al,Transvaginal ultrasonography in predicting placenta previa at delivery: a longitudinal study.Ultrasound Obstet Gynecol. 2002 ;20(4):356-9. PMID: 12383317
3.Dashe JS, et al. Persistence of placenta previa according to gestational age at ultrasound detection. Obstet Gynecol. 2002;99:692-7.  PMID: 11978274
4. Reddy UM, etr. al. Fetal imaging: executive summary of a joint eunice kennedy shriver national institute of child health and human development, society for maternal-fetal medicine, american institute of ultrasound in medicine, american college of obstetricians and gynecologists, american college of radiology, society for pediatric radiology, and society of radiologists in ultrasound fetal imaging workshop. J Ultrasound Med. 2014 May;33(5):745-57. 
PMID:24764329