With pregnancy tests being negative, chance of pregnancy is very low if you have done the tests in the second week of missed period. In the first week of missed period, the blood pregnancy test is more sensitive and home pregnancy tests can come negative as pregnancy hormone built up is not enough to be detected by the home urine pregnancy tests.
The abortion pill called as RU486 is a prescription product and is taken under medical supervision, so one if your pregnancy is confirmed, you will need a doctor's visit.
Apart from pregnancy there are other causes of missed or delayed periods, also, so they must also be considered;
1) Ovarian hormone dysfunctions
2) Thyroid hormone abnormalities
3) Stress, anxiety, emotional disorders
4) Sudden weight loss or gain
5) Excessive physical activity 6) Hyperprolactinemia
Read the resource under the heading secondary amenorrhea;