Your LMP was on 8th Dec
.So your ovulation occurs around 22nd Dec (unsafe 18th-26th Dec) or 24th Dec (unsafe 20th-28th Dec) if previous cycle was regular.
You did unprotected sex without ejaculation inside on 18th Dec,near about ovulation time or risk zone.So chance of pregnancy
is minimal (as pre cum/pre prostatic fluid,secreted prior to actual ejaculation, contains sperm and causes pregnancy sometimes).
You need to take emergency pill (I-Pill) immediately within 72 hrs of sexual intercourse as after that it has no effect on preventing unwanted pregnancy.
So wait for the next coming period and if missed,do a urine test for hcg/pregnancy test after 7 days.
Check result and consult with your gynecologist to take suggestive measures.
Practice safe sex
in future. Be well.