Why do some women have menstruation-like spotting throughout their pregnancy?
Here are some causes of bleeding during pregnancy:


early pregnancy (first trimester):

implantation bleeding [1]
infection [1]
after intercourse [1]
miscarriage [2]
ectopic pregnancy [2]
cervical changes [3]

second part of pregnancy:

abruptio placentae [1, 2]
placenta previa [1, 2]
miscarriage [2]
vasa previa [2]
preterm labour [1]





References:


American Pregnancy Association. Bleeding During Pregnancy. Available from http://americanpregnancy.org/pregnancycomplications/bleedingduringpreg.html (accessed 29.07.2014)
U.S. National Library of Medicine. A.D.A.M. Medical Encyclopedia. Vaginal bleeding in pregnancy. Available from http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0003748/ (accessed 29.07.2014)
WebMD, LLC. Bleeding During Pregnancy. Available from http://www.webmd.com/baby/guide/bleeding-during-pregnancy (accessed 29.07.2014)