Implantation spotting is a cause of anxiety and confusion for many women, whether actively trying to conceive or not. It can be taken falsely for the onset of a period, or it can equally falsely be taken for the beginning of a miscarriage. Whatever your circumstances, it's good to understand what is going on. So what's it all about?
If you can remember your high school biology, you'll know that after ovulation, the egg travels down the fallopian tubes, where it may be fertilised by a helpful sperm before continuing its journey down to the womb. If fertilisation occurs, the embryo may implant in the womb lining and create a pregnancy.
So far so good. But why are you bleeding? Very simply, because your baby needs to access your blood supply in order to start the process of building a viable pregnancy: while the first connections to your body are made by the chorionic villi, these will later become a placenta that will tap into your circulation to ensure that the baby gets nutrients and all kinds of other good stuff like antibodies from you. Naturally enough, as the fetus burrows into the womb lining, it'll do a little damage, which causes bleeding. Not vast amounts, but this is what you'll have noticed as implantation spotting. In many cases this goes completely unnoticed, the quantities can be so small, while in other cases there may be as much as a teaspoonful of blood. Usually, though, the blood is pinkish or brown in colour rather than bright red, signifying that this is old blood and hence nothing to be concerned about. It may, however, last 2-4 days, which is perhaps why some people confuse it with the onset of a period, assuming that it was just very light that month. This is further complicated by the fact that niggling, period-like cramps are not uncommon in early pregnancy, so a woman with a history of very light periods could easily assume she was not pregnant.
When does this bleeding occur? Generally around the 4-5 week mark, though this may vary if you tend to have longer cycles. If the bleeding occurs more than a couple of weeks later than this, however, or if the blood is red and needs you to use a pad, it's worth getting checked out by a doctor. There are many things that can cause a bleed during pregnancy, especially in the early stages, where there is so much blood flow to the pelvic area to gear the body up, especially to the cervical area, which often bleeds perfectly harmlessly after a rough internal exam or indeed after enthusiastic intercourse. In fact, if you need to have a transvaginal ultrasound scan, this may also cause some bleeding, but this is nothing to worry about, and the sonographer may even warn you to expect this.
The key thing to note about implantation spotting, however, is that it's generally a good sign for the pregnancy, even if the least sign of a bleed has you thinking it's all over. I know from personal experience how worrying it can be when a much longed-for pregnancy seems threatened by unexplained bleeding. Whatever else you do, make no assumptions about the pregnancy's viability on the basis of implantation bleeding alone. If there's a chance you may have conceived and such bleeding as you had was not normal for a period, you should always consider taking a home pregnancy test to make sure, whether the pregnancy is wanted or not.
Finally, while implantation spotting is generally good news for a pregnancy, there are some circumstances in which you should always seek medical attention. If you experience sharp, stabbing pelvic pain which increases when you touch your abdomen (unlike period pain), or if you start getting sharp referred pain in your shoulder, accompanied by the implantation bleeding, this may be a sign that implantation is occurring in the wrong place, resulting in an ectopic pregnancy. This is extremely dangerous for the mother and such a pregnancy is not viable, so medical attention should be sought immediately. It's always better to be safe and, while the internet can provide plenty of information, it's no substitute for the advice of a good doctor who knows you and can examine you in person.