Thanks for using healthcaremagic.I understand your concern. Brief answer: Posterior low lying placenta usually won t cause harm to a growing fetus.But this condition needs to be carefully monitored by an obstetrician. Detailed answer: During early pregnancy(up to 20 weeks)--Anomaly ultrasound scan,serum marker assay(Triple or quadruple marker test),amniocentesis---helps only to detect certain genetic abnormalities like Down syndrome etc During later pregnancy(After 26 weeks)--Non-stress test and contraction stress test using cardiotocography,Lecithin/Sphingomyelin ratio to assess fetal lung maturity,amniotic fluid index(AFI),biophysical profile test(BPP),Doppler velocimetry---can be used to monitor the fetal well being. For high risk pregnancies,growth ultrasound every month can be used particularly where growth restriction is suspected. Also you need not be worried regarding the ultrasound at present,as the posteriorly lying placenta may not be seen near the internal os in future ultrasound scan when uterus tends to grow further.I would advise you to follow up with your obstetrician,who is expertise in monitoring and taking care of your unborn with the necessary tests from the above mentioned.Hope this information helps you.If any other questions,happy to answer you.Take care.