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Having blood type O negative when pregnant can lead to some complications. If a person with this blood type gets pregnant, the mother's body may produce antibodies that can attack the baby's red blood cells. The baby may have symptoms like jaundice or anemia. Doctors can give a medication called Rh immunoglobulin to the mother during pregnancy to prevent this. 

It will be your OB-GYN who will figure out the exact date when conception took place. She will provide further guidance during your pregnancy. It is of crucial importance to see a competent specialist, if there is any question of a problem related to pregnancy.

In this case you should go for second opinion of another experienced doctor. They can evaluate your case and then you can come to a conclusion.

Not having periods when they are due, throwing up, being weary, sensitivity in the chest area, and frequent urination may indicate that someone is pregnant. Feeling like one is pregnant might also come from anxiety, hormonal imbalances, or other medical conditions. Make sure to use a pregnancy kit. 

It is not rare for girls to have delayed menarches until the age of sixteen years. Although, having pain in the lower abdomen might be a red flag and will need to consult with any gynecologist. To have an overall reproductive health, it is essential that one gets proper diagnosis and treatment from the specialised doctors.