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Why is there a discrepancy in calculating my pregnancy days?

Q: I am a 33 years old woman in the seventh month of my pregnancy, which is about 26 weeks. But my scan report shows it to be about 28 weeks and this change of two weeks growth is from the beginning itself. My BP and other parameters are in normal condition and the baby's movement is also adequate. Is there any problem in this case?

A:I am not sure why ultrasound (US) was done at 6 months. It is rarely required at this stage, so please ask your doctor if any specific complication was expected and why Ultrasound was ordered. I also want you to understand that US does not tell you how many weeks your pregnancy is. That is calculated according to the day of conception, which depends on things like your date of last period and the length of your menstrual cycle. US merely estimates the size of the parts of the fetus in millimetres and this is analysed by a software to approximately calculate the age of the fetus. All these calculations are at best estimates and not very accurate. If there is a discrepancy of over 4 weeks between the age calculated by dates and that by US do we need to consider it abnormal. And we are more worried if the baby is too small rather than too big. Anyway the commonest cause of this kind of discrepancy by a week of two is error in calculations. First the dates should be definitely known, and then when we convert months to weeks there should be no error. For example: 6 months are for 26 weeks not 24, i.e. each month has 4 weeks plus 2 or 3 days (except February) and these need to be accounted for in the calculation of age in weeks. More over we calculate the age in weeks from the first day of the last period recorded assuming it is a 28 day cycle. So, for example, the age is one week more in women with 21 days cycle and one less in a woman with a 35 days cycle. Once you have been careful and excluded all these possible errors, if the baby is still larger you need to consider conditions in which the fetal growth is more than expected. That is also mostly due to genetic and racial factors (both parents are above average in height and weight). If that is also not the case, the major factor to be ruled out is a high blood sugar level/diabetes. Even blood sugars considered normal for a non-pregnant adult may be a problem in pregnancy. So you need a blood sugar estimation (fasting and one hour after each meal) to see that your blood sugars are not high. If that is normal as well just see your doctor and the commonest problem is wrong dates. Anyway if the discrepancy has been there since early pregnancy it is unlikely that it is due to a medical cause and it must be because of a calculation error and you do not have to worry at all. Besides, the problem is only if there is a discrepancy of 4 weeks or more.

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