Will I get pregnant after taking post pills?

At 12 weeks, the 2 cm head size of the baby displayed in the scans is normal. This is nothing but a predictable growth pattern. The baby's brain will continue to grow to the same body proportions during the pregnancy. An infant's cranial size at this stage is crucial in the development of the brain, thus this is an important parameter to control. 

if you have at risk sex close to your period, delay or wait.
There is an option for emergency contraception pills, however they are not 100%.
Use condoms to avoid pregnancy and getting sexually transmitted infections.
Consult a physician for further advice and routine follow-up.s

If you've had sex without protection, you are at high risk of getting pregnant, so don’t miss taking contraceptive pills but it is better to talk to your doctor once

If you had protected sex and have missed your period, there could be many reasons for a missed period other than pregnancy. Stress, changes in weight, illness, hormonal imbalances, and various medical conditions can all affect your menstrual cycle. Please get a test done to confirm about pregnancy.

Beyond 20 weeks pregnancy terminations is not recommended. If you still have any doubt it's critical to visit a gynecologist or an obstetrician for the process and medical services required. Self-medication or attempting a home abortion without a doctor's supervision is not recommended.