Why do some women have menstruation-like spotting throughout their pregnancy?

Question

An interesting question that was asked during a lecture about human reproduction: what is the biological mechanism behind some pregnant women getting monthly spotting that they mistake for a regular period? I've found articles and research papers about bleeding during zygote implantation, ectopic pregnancies and problems with the placement of the placenta, but nothing regarding menses-like bleeding during otherwise unproblematic pregnancies.

Answer 1

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]

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References:

1. American Pregnancy Association. Bleeding During Pregnancy. Available from http://americanpregnancy.org/pregnancycomplications/bleedingduringpreg.html (accessed 29.07.2014)
2. 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)
3. WebMD, LLC. Bleeding During Pregnancy. Available from http://www.webmd.com/baby/guide/bleeding-during-pregnancy (accessed 29.07.2014)