20-Week Ultrasound Accuracy: Dating & Due Date - Expert Advice

Question
i was wondering how accurate the 20 wk ultrasound is on the dates of conception and due date? when they measure the baby and stuff,is is accurate if the measurments match the due date based on your LMP? because i wanted to make sure i didnt concieve 2days before my period,especially with my cycle being 35 days long...{my last LMP was Feb.13,2006}


Answer
You cannot conceive before  your period. If your last period started on February 13, and you have 35 day cycles (from the first day of your period until the first day of your next period)ovulation occurs on cycle day #21. If your last period started on February 13, ovulation would have occurred on March 5 and  you could have conceived if you had sex on March 4,5,6, or 7. You are due on November 20.  The 20 week sonogram is highly accurate.