How to Calculate Calendar Weeks

Pinpointing the exact moment of conception may be difficult without the use of ultrasound, but a fairly accurate estimate can be made using the first day of your last menstrual period (LMP) and a calendar. Doctors routinely calculate due dates and pregnancy milestones using simple counting skills and your answers to a few questions.



A full-term pregnancy ranges from 38 to 42 weeks, with only 5 percent of babies leaving their mommy-motel on day 280. The other 95 percent are born either before or after the 280-day mark.

Things You'll Need

• Calendar
• Highlighter or pen

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Instructions

• 1

  Determine the first date of your LMP, which is used to indicate the day that your pregnancy began. Most women can't predict the exact moment of conception, so the LMP -- before ovulation and fertilization -- is used to define the beginning of a pregnancy.

• 2

  Count forward from your LMP on the calendar. Use the exact day of the week moving forward to count off the weeks on the calendar. For example, if your LMP was on a Monday, use the Mondays of each week to count milestones.

• 3

  Put a special mark on week 12 from your LMP to indicate the end of your first trimester and on week 20 to signify the midpoint of your pregnancy. Continue counting weeks until you reach week 28 at which point your baby is viable and has an excellent chance of living outside the womb. Week 37 is the milestone generally accepted as full term, and your baby can be born at any time within the next 21 days.