You just have to pay attention to your body and willing to have extra fun.  Knowing your menstrual cycle is very important; Ovulation is always one week after the last day of your period.  Try having sex more often then for quick results.

My last period was from the 8th to the 13th of every month, so I was ovulating from the 19th to the 25th of March, which was around the time I conceived. I missed my period in April and found out I was 5 weeks pregnant on April 17th.

I knew I was pregnant at 2 weeks, I just felt very tired and sleepy.  I couldn't tell anyone because I was not sure, I didn't want to buy a home pregnancy test because  didn't want to be disappointed.  So i didn't want to think about it, 2 weeks later I still had a cold. 
A week and a half later I went to see my doctor for a blood test and a week after that the confirmation via phone and I was 5 weeks pregnant.  I know this is funny  but ......I still went ahead and bought the home pregnancy test at around 11 weeks pregnant....positive...I am 15 weeks and 6 days today!

  .....As long as there are no health problems related, I think it is fine to have a big or small baby....I guess it is just a bit easier to push at birth.