Chance of pregnancy
is high enough, if unprotected sexual intercourse is done with ejaculation around ovulation time in regular period. Ovulation occurs approx 14 days before due date.
In your case, period due date was 6th January. So ovulation occurred around 23rd December. You did unprotected sex on 17th Dec which was almost safe period or having low risk as compared to ovulation time.
As your period is more than 10 days late, it is better to a urine/ blood test
for hcg to confirm/ exclude pregnancy. Check the result and consult with gynecologist.
Other reasons like hormonal
problem, wt gain/ extreme loss,irregular eating habits etc. are required to rule out if pregnancy test is negative.
Avoid stress,take healthy diets, control body wt and need proper sleep.
If you need further consultation with me,then you can visit http://www.healthcaremagic.com/doctors/dr-soumen-patra/63326.