The two ultimate pregnancy symptoms are a missed period and a positive pregnancy test. Menstruation is, after all, the body's way to shed your uterine lining in preparation for a possible pregnancy during the next cycle if you didn't get pregnant this time . It is not possible to experience a period when you are already pregnant.
Why do so many women ask questions just like this one, then?
Can I've one day period throughout the month after I'd sex with my husband during my ovulation and don't get any signs of pregnancy in me?
If you think you've experienced a period despite being pregnant there are, in short, two explanations:
- You are indeed pregnant, and the bleeding you interpreted as a period was actually something else.
- You really did get your period, and you are not pregnant.
Causes Of Early Pregnancy Bleeding That You May Mistake For A Period
Between seven and 24 percent of newly pregnant women are estimated to experience vaginal bleeding during the first trimester. While this is certainly worrying to those women who are already aware that they are expecting a baby, early pregnancy bleeding doesn't have to signify an impending miscarriage. In fact, a surprisingly low 12 percent of those women who experience vaginal bleeding during their first trimesters will miscarry, compared to 13 percent of those who do not have any bleeding, one study suggested. .
What causes early pregnancy bleeding, then? Women over 35 and those who were exposed to passive smoking during their pregnancies are more likely to experience bleeding, as are those with a history of miscarriage and those who had multiple surgical abortions . Women who suffer from bacterial vaginosis and other pelvic infections are also at an increased risk of experiencing vaginal bleeding early on in their pregnancies .
Another possible explanation is a so-called implantation bleeding that is thought to occur in some women when the new embryo nestles itself into the uterine lining . While scientists are still not quite sure when this implantation occurs, it is believed to take place somewhere between eight and 12 days following conception — as signified by the presence of detectable levels of human Chorionic gonadotropin, the hormone pregnancy tests react to. 
This means that those women who notice an implantation bleeding are most likely to experience it shortly before their period was due to arrive. However, it is possible that those who have slightly irregular menstrual periods interpret an implantation bleeding as an unusually short and light period. (Though little scientifically accurate data is available regarding the duration and severity of implantation spotting, anecdotes from women who believe they experienced such bleeding suggest that an implantation bleeding tends to last up to two days, and that it is much lighter than a menstrual period.)
So, I Think I Could Be Pregnant — What Should I Do Now?
Home pregnancy test manufacturers widely claim an accuracy rate of 99 percent if the test is taken on the day of your missed period or after. While you should be aware that the hormone detected by these tests, hCG, tapers off after about 12 weeks gestation, it will continue to be present throughout your pregnancy.  Still, if you have come to believe you are pregnant despite previously thinking you had a period, you are likely to be taking a test later on in your pregnancy than most women do, and your hCG levels may be lower.
You can also choose to ask your doctor for a blood pregnancy test — should you not be pregnant, and your belief that you may be stemmed from unusual vaginal bleeding, seeing a doctor is in order anyway. Other causes of abnormal uterine bleeding include hypothyroidism, liver disease, uterine polyps or fibroids, and pelvic infections. Should your unusual bleeding be caused by any of these factors, you will certainly want to receive medical treatment.