Many women wonder if cramping is an early sign of pregnancy. Cramping can be sign of pregnancy, and sometimes it isn’t a sign of pregnancy. Like most early signs of pregnancy, cramping can also mean something else. For most women who are not pregnant, cramping is a sign of a period coming on.

When cramping is a sign of pregnancy, it can be confusing. There are many reasons why women experience cramping during this time in pregnancy.

Implantation
Some women feel cramping when the embryo is implanting itself into the lining of the uterus. This usually happens between 6 and 12 days past ovulation, or about 3 weeks after your last period began. 

Early Pregnancy Changes
Cramping as an early sign of pregnancy may also occur because the uterus is undergoing rapid changes to accommodate the pregnancy. The uterus must stretch and grow to make room for the developing baby and placenta. There is more blood flow to the uterus and it doesn’t take long to grow from the size of a fist to the size of a grapefruit. Many women think that their period is about to arrive at any time when it is really cramping due to these changes. If you chart your morning temperatures every day, you may already suspect you are pregnant if your temperatures are staying in the higher range. In this case, early cramping may either confirm your suspicions or cause you to worry about losing the baby. Cramping by itself usually isn’t a problem, and you can look at it as a good sign. If it is accompanied by bleeding, you should contact your care provider.

If cramping is accompanied by other possible signs of pregnancy  like nausea, tender breasts, fatigue, sensitivity to smells, a change in vaginal discharge, and urinary frequency, congratulations! As you can see, cramping is often felt in early pregnancy, and may actually be an early sign. Patience is the best way to figure out what your cramping is telling you. Many women will experience cramping from early on in their pregnancy through most of the first trimester. 

Source : summarize from  Susan Baler