The Count-Back Method

• 1). Work out the start date of your last menstruation. For example, Sept. 1.
  
  
• 2). Add seven days to this date: Sept. 8.
  
  
• 3). Count back three months from the new date: June 8. (If your start date falls after March, you will need to add one year to the final date.)
  
  

The 40-Week Method

• 1). Calculate the start date of your last menstruation. For example, Sept. 1.
  
  
• 2). Add 40 weeks to this date: June 1.
  
  
• 3). This is your estimated due date. The main problem with this method, however, is that it bases the pregnancy on women who have a 28-day cycle and doesn't take into account individual differences.
  
  

The 266-Day Method

• 1). Estimate the length of your menstrual cycle. For example, 30 days.
  
  
• 2). Estimate the date of conception, taking into account your personal menstrual schedule. For example, 15 days = Sept. 15.
  
  
• 3). Add 266 days forward from the point of estimated conception: June 8. (This method takes into account that there may be a few days' leeway on either side of the estimated conception date.)