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Calculating your baby's due date

Calculating your baby's due date


Once expectant parents have a 'positive' pregnancy test, their first question often is "When is my baby due?"



Caregiver's will refer to the baby's due date as the 'estimated date for delivery' ('EDD') or 'estimated date for confinement' ('EDC'). However, it is precisely that, an ESTIMATE! Even if your baby's 'due date' is accurately calculated from the definite day they were conceived, there is still only a 5% chance they will be born precisely on their due date!

Because every woman and her baby are different, we really don't know how long it takes each individual mother to 'grow her baby'. What we do know is that it is normal and healthy for babies to be born at any time from about 37 to 42 weeks of the pregnancy. However, even though babies born during this 5 week period are fully mature and ready to be born, they still may be 3 weeks earlier or two weeks later, than the estimated 'due date'. (Babies born less than 37 weeks are regarded as 'premature' and babies born later than 42 weeks are regarded as 'overdue'.) Most babies are born a week either side of the estimated due date, but are more likely to come the week after the due date, rather than the week before. 

Calculating the date

Pregnancy due dates are said to be 40 weeks long. The '40 week' estimation was first developed by a German obstetrician called Naegele in the 1800's (hence it is called 'Naegele's Rule'). He declared that a woman's pregnancy should last 10 lunar months (or 280 days), being about 9 calendar months. However, because Naegele used the first day of the woman's last menstrual period as a definite 'starting point' to base his calculations on, and conception typically happens 2 weeks after this time (or two weeks before the woman's next period is due) a pregnancy is really only 38 weeks long (or 266 days), being about 8 ½ calendar months. Therefore, one week after conception, a woman is said to be '3 weeks pregnant'. Caregivers today still use Naegele's rule to calculate a baby's due date.

Naegele based his calculation on the woman having a regular 28 day menstrual cycle. Therefore, if you have a regular 28 day cycle and you know the date of the first day of your last normal period, you can use Naegele's rule to calculate your baby's due date, which is

9 months + 7 days after the first day of your last NORMAL menstrual period (or 280 days after this day).

You can do this calculation quickly by using our Due date calculator, which is based on Naegele's rule.

However, not all women have a regular 28 day cycle. Therefore, if your cycle was longer or shorter than 28 days, Naegele's rule needs to be adjusted. You can do this by first using Naegele's rule and the first day of your last menstrual period to find the date, then add or subtract the appropriate amount of days, depending on your cycle. For example:

  • If you have a 26 day cycle you would subtract 2 days from the Naegele's estimated due date.OR
  • If you have a 32 day cycle you would add 4 days to the Naegele's estimated due date.

Some women are not sure about the date of their last menstrual period, but are certain of their baby's conception date. To calculate your baby's due date from their conception date, you add 266 days to your conception date. (This is essentially the Naegele's rule date, minus 14 days, or 9 months minus 7 days after the first day of the last normal period). Bear in mind that it is possible to have sex 3 to 5 days before you ovulate (or release an egg), making your 'conception date' 3 to 5 days later than the day you had sex (unless you are sure about the day you ovulated).

Factors that can alter the date

The baby's due date can be miscalculated if the woman experiences an 'implantation bleed'. This is when the growing baby implants in the blood-rich lining of the woman's uterus, about 8 to 12 days after conception (or about 2 to 6 days before the next period would have been due). An implantation bleed can sometimes be confused with being the 'last normal period' (although an implantation bleed is usually lighter and does not last as long as a period). Caregivers will try and confirm what your last period was like, so that the baby's due date is not calculated from an implantation bleed. (Otherwise the due date will be estimated as being about 3 to 4 weeks later than it should be).

In some cases, the starting date of a woman's last period and/or her baby's conception date is unknown. This may be because she has very irregular periods, or her natural menstrual cycle has been disrupted because she has recently stopped hormonal contraception, or is breastfeeding or has recently experienced a miscarriage. (These are discussed in depth in variations for conception.) The caregiver may then be guided by an ultrasound calculation.

Ultrasounds can be of great help in determining how far pregnant a woman is, particularly if she has no idea herself. However, ultrasounds are not always accurate at calculating the gestation of a baby, because they rely on measuring the baby's physical size. (Their 'crown to rump' length during early pregnancy and their femur (or thigh) length and head size, as they grow older.) 'Due date' estimations are based on the 'middle average' size of babies, but individual babies differ in size depending on their genetic make-up (as all human beings do). Therefore they are not always accurate. Generally speaking:

...the earlier the ultrasound is done, the more accurate it can be at estimating the baby's due date.

However, not all women have a regular 28 day cycle. Therefore, if your cycle was longer or shorter than 28 days, Naegele's rule needs to be adjusted. You can do this by first using Naegele's rule and the first day of your last menstrual period to find the date, then add or subtract the appropriate amount of days, depending on your cycle. For example:

  • If you have a 26 day cycle you would subtract 2 days from the Naegele's estimated due date.OR
  • If you have a 32 day cycle you would add 4 days to the Naegele's estimated due date.

Some women are not sure about the date of their last menstrual period, but are certain of their baby's conception date. To calculate your baby's due date from their conception date, you add 266 days to your conception date. (This is essentially the Naegele's rule date, minus 14 days, or 9 months minus 7 days after the first day of the last normal period). Bear in mind that it is possible to have sex 3 to 5 days before you ovulate (or release an egg), making your 'conception date' 3 to 5 days later than the day you had sex (unless you are sure about the day you ovulated).

As a guide:

  • Ultrasounds performed during the first 12 weeks of pregnancy are generally within 3 - 5 days of accuracy. This is because the baby is growing extremely rapidly, and there is a great deal of difference between the sizes of a 7, 8 or 9 week old unborn baby. Some caregivers will recommend an early ultrasound to help calculate the woman's due date (however, if you are sure of your dates, then a dating ultrasound is not necessary). If you have an ultrasound done before 12 weeks of pregnancy and your baby's age is estimated to be more than +/- 7 days from the original due date, your caregiver will probably readjust when your baby is due.
  • As the baby grows, the differences in their growth stages from week to week become less obvious, reflecting the individuality of each woman's baby. Ultrasounds from 12 to 22 weeks are regarded as being within 10 days of accuracy (or up to 10 days earlier or 10 days later than the woman's calculated due date). If you have an ultrasound during this time and your baby's age is estimated to be more than +/- 10 days from the original due date, your caregiver will probably readjust your due date.
  • Ultrasounds performed after 22 weeks can be up to 2 to 3 weeks out, and as a general rule should not be used to estimate the due date of your baby (unless this is all you have to base your due date on). If you have more than one ultrasound during your pregnancy, giving you 'multiple dates', then the earliest ultrasound estimate should be used.

NOTE: If you are sure about when you conceived and/or know the first day of your last menstrual period (and having a pretty regular menstrual cycle), this is more accurate than any ultrasound measurement!


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