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In many cases, it can be difficult to tell the difference between PMS and pregnancy because the symptoms are so similar. Symptoms of pregnancy are often noticeably worse than symptoms of PMS, but not always. The only true way you can tell the difference between PMS and pregnancy is to wait for a missed period, after which time you may take a pregnancy test to be sure.
Some of the symptoms that are common between both PMS and pregnancy include breast tenderness, irritability and mood changes, fatigue, headaches, and cramps. Because each woman experiences PMS differently, and some do not experience it at all, it should be most noticeable to you if something seems different one month. Usually, the symptoms associated with pregnancy are much more pronounced; for instance, rather than being just slightly tired, you might feel absolutely exhausted and need to take a nap in the afternoon.
In addition, breasts may appear to be very swollen and much more painful than usual. You might start getting severe headaches, or notice that certain foods or smells become very unappealing. In general, nausea and vomiting is not associated with PMS; if you are experiencing these symptoms, this is a fairly good indication of pregnancy rather than PMS. Conversely, though you may experience some mild stomach cramps, you should not experience severe stomach cramps during early pregnancy.
Otherwise, the best way to tell the difference between PMS and pregnancy is to listen to your body. The symptoms of PMS and pregnancy are very similar, but if you know how your body usually responds every month, you will often be able to tell if something is different. Fortunately, the symptoms will generally only last about one week before you know if you are pregnant, or just have PMS. If it was just PMS, the symptoms will disappear; if you are pregnant, however, the symptoms will continue, and will likely become more pronounced.
It can be difficult to wait to find out if it is just PMS or pregnancy, but most pregnancy tests cannot be taken until you actually skip a period because they are simply not sensitive enough to work any earlier. Once you have taken a pregnancy test and it comes back positive, you should visit a doctor for a blood test to be sure. Some women experience spotting that can be mistaken for a period in early pregnancy, so it is important to continue to monitor the progression of your symptoms if there is a possibility you could be pregnant, and to adjust your diet and other habits accordingly until you are sure.