While the due date of a baby is actually only a rough guide for predicting when your baby will be born, many parents-to-be are eager to have a date to look forward to in anticipation of their new arrival. An estimated due date is also important for deciding when to carry out some prenatal tests, since these tests are most accurate during certain time periods during gestation. Traditional dating methods use calculations to estimate your baby's future birthday, but ultrasound estimations, which take into account the growth of the baby in the womb, are typically more accurate. Sometimes, if the estimated date doesn't match with certain pregnancy milestones, a doctor will change the due date.
Step 1
Determine the specific date of the first day of your last menstrual period. If you were charting your menstrual cycles, you may know the precise date. If unsure of the exact date, try to get as close as possible.
Step 2
Subtract three months from the date you determined. For example, if your last menstrual period began on April 3rd, three months before that would be January 3rd.
Step 3
Add seven days to that date. For example, if the three month subtraction gave you January 3rd, adding seven days would give you January 10th. This is your due date as defined by traditional methods. Typical gestation is 280 days, or 40 weeks.
Step 4
Consider your cycle. If your menstrual cycle is not 28 days, but is regularly some other number, you may be able to get a more precise date by adding or one day for each day longer that your cycle is or subtracting one day for each day shorter than 28 days your average cycle lasts. For example, a woman with a 30-day cycle would add two days onto her calculated due date, making it January 12th instead of January 10th, while a woman with a 27-day cycle would subtract a day, estimating her due date at January 9th.
Step 5
Get an ultrasound to get a more precise estimation. According to the journal Obstetrics and Gynecology, determining the gestational age and calculating the due date based on ultrasound measurements of the fetus is more accurate than using the last menstrual period as a guide, as long as the ultrasound is performed before 18 weeks gestation. Essentially, the earlier the ultrasound is done, the better the estimation.
Tips and Warnings
- Don't focus too much on your due date, but use it as a guide. According to the University of Maryland Medical Center, only five percent of women have their baby on their exact due date and seven percent go more than two weeks past their due date. Most pregnancies last between 37 to 42 weeks and anything within this time frame is considered normal.
Things You'll Need
- Calendar
- Date of last menstrual period
- Prenatal ultrasound appointment
