Which EDD is more accurate?

Question

The due date you get from the ultrasound
or
counting 40 weeks from your last cycle?

Answer 1

Doctors seem to change their minds all the time especially if you see a few different doctors. I am having the same problem at the moment as I went into premuture labour and the doctor has now moved my due date even further away then it should be. If you know your last period and are sure then use that as the best but it may vary after an early ultrasound. Remember that every baby grows at a different rate so the ultrasound is just an estimate if done after the first trimester.
I have some infomation here that may help you understand. Good luck and I hope it helps.

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. This is discussed in depth in post dates.



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).

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 (or our BIRTH calculator ) 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.


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!

Answer 2

If you know for sure your last cycle, then I personally would use that, but they do use the ultrasound to date pregnancy. If there is a big discrepancy, I would discuss with my doctor.
With the Ultrasound they told me I was having a really big baby...she wasn't.. so that was off, but that is different than dating a pregnancy. Point being I think they can be wrong.

Answer 3

Is there a really big difference between your ultrasound date and your last period? The ultrasound measurements can be off by a week or two, so I would say go by your last period but I would also ask your doctor what to go by in this case.

Answer 4

In many ultrasound departments, a common practice is to combine the last menstrual period (LMP) with ultrasonographic dates in what is known as the 10-day rule or 7-day rule. For example, with the 10-day rule, if LMP dates and ultrasonographic dates are in agreement within 10 days, LMP dates are accepted. On the other hand, if the discrepancy exceeds 10 days, ultrasonographic dates are used. The rationale for using these rules is to exclude large errors from incorrect menstrual dates. The implicit assumption of this method is that menstrual dating is preferable to ultrasonographic dating. Detailed analysis from large databases has not shown any advantage in using these rules (Mongelli, 1996). Unless the fetus is thought to be anatomically abnormal, ultrasonographic dates may be used for all pregnancies if a scan is available in the first half of pregnancy. However, combining menstrual dates with ultrasonographic dates is still widely practiced in the United States and in Australia
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Answer 5

EDD stands for estimated due date unless you know the exact date you became pregnant the ultra sound would be more accurate they measure the baby's femur bone to try to estimate intrauterine growth...remember though you could go 2 weeks before or 2 weeks after your EDD, perferably not going into labor before 36 weeks

Answer 6

With my first being pregnant my EDD change into precisely fantastic with ultrasound (I had it at 8 weeks). i change into also charting besides so i change into very virtually one hundred% particular that the due date change into fantastic. I had my newborn 2 weeks previously my EDD. i really do experience for you. i understand the finest days of being pregnant are quite confusing!! with any success you get to satisfy your toddler truly quickly :)

Answer 7

I would say that your last period...BUT...

The first ultrasound (at around 8 weeks) is extremely accurate.

Answer 8

I used several methoda to count and at the end none was reliable. But with my 1st, I counted manually and it gave me the exact date at which my constructions started, though I gave birth only the next day. But I knew exactly when I'd conceived.

Answer 9

By going by the first day of your last period

Answer 10

LMP