What Causes Delay Periods?

My wife and I last had sex on 12/5. My wife has had two menstrual periods 18 days apart 12/7-12/10 and again 12/25-12/28 (normal and on time). She took a pregnancy test on 1/16 and it was negative. She also started birthcontrol around 1/3. Is the birthcontrol making her cycle late? It's now January 21st.