How Do I Calculate Due Date by the Baby's Actual Age?

Approximately 5 percent of babies actually arrive on their true due date.

A normal healthy pregnancy lasts 280 days, or 40 weeks. Your baby's estimated date of delivery, or due date, is 40 weeks following the exact date of conception. Since many women do not know their exact date of conception, gestational age and due dates tend to be approximate. Gestational age is the exact number of days and weeks since your baby was conceived. Babies seldom arrive on their due date, but most are born within a week or so of that date.

Things You'll Need

• Calendar
• Pencil
• Calculator
• Computer with Internet access

Instructions

If You Know Your Actual Date of Conception

1. Mark your actual date of conception on the calendar.

2. Calculate actual gestational age by counting days and weeks since conception. Midwives and doctors abbreviate this in a weeks plus days format, i.e. 14+5 is 14 weeks and five days gestation.

3. Count forward on the calendar until you arrive at 40 weeks. This is your calculated due date. Your baby should be born sometime within seven to 10 days of this date.

If You Don't Know Your Conception Date

1. Calculate the average length of your menstrual cycle. Look up the date your last period began. This will give you a starting point for your calculations. If you don't know the average length of your cycle and you keep accurate cycle records, you should be able to count days from the beginning of one period to the beginning of the next period. If your cycles are not regular, choose the average length. Cycles average 28 days, but yours may be longer or shorter than that. Accurate cycle length will provide the most accurate due date calculation. If you don't know the first day of your last period, think about the events around your last period and try to approximate the date.

2. Calculate what day you normally ovulate in a cycle. Some women experience a sharp pain directly over an ovary when an egg is released. This pain is called "mittelschmerz." Count the days between the first day of your period and mittelsmerz. This is the day you normally ovulate. This number is also the length of your luteal phase. If you do not experience mittelsmerz, subtract 14 days from your average cycle length.

3. Subtract the length of your luteal phase from the date of your last period. This will give you the probable date you ovulated. Then, pinpoint any time within 72 hours of that date when you might have had sexual intercourse. This includes 24 hours before you ovulate and up to 48 hours after ovulation. This should give you the probable date of conception.

4. Count the number weeks and days since conception to arrive at your baby's true gestational age.

5. Add 40 weeks to your ovulation date or count forward from your baby's gestational age to reach 40 weeks. This is your baby's estimated due date. Remember that most babies do not arrive on their true due date. This date is the approximate birthday of your unborn baby.

Tips & Warnings

• If your calculations do not match the due date predicted by a sonogram, don't worry. Sonograms determine due date by the growth and development of the unborn baby. There is more variability in growth and developmental rates as the pregnancy advances. Hence, due dues calculated from first trimester sonograms will be more accurate than those done later in the pregnancy.

References

• Mayo Clinic: Mittelsmerz
• Natural Family Planning: Information Site
• Due Date Calculator