Updated on 10 May 2023
Pregnancy is an exciting time, but it can also be overwhelming, especially when it comes to determining the due date of the baby. Gestational age is a term used to describe the age of the baby in weeks and days from the first day of the last menstrual period (LMP). The due date is calculated by adding 40 weeks or 280 days to the first day of the LMP. However, the due date can also be determined by ultrasound, which measures the size of the baby and calculates the due date based on that measurement.
In this article, we will understand the possible causes and implications of discrepancy between LMP and ultrasound gestational age and what is more accurate ultrasound due date or LMP.
The LMP is used to determine the gestational age of the baby because it is the most accurate way to determine when the pregnancy began. The LMP is the first day of the last menstrual period, and it is used to calculate the due date by adding 40 weeks or 280 days. Ultrasound gestational age, on the other hand, is determined by measuring the size of the baby. The ultrasound technician measures the baby from head to bottom, and the CRL measurement is used to calculate the due date.
There are several reasons why there may be a difference between LMP and ultrasound gestational age.
One of the most common reasons is that the LMP may not be accurate. Women may have irregular menstrual cycles, making it difficult to determine the first day of the last menstrual period.
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Some women may experience implantation bleeding when the conceive. Implantation bleeding is light spotting that may occur when a fertilized egg implants itself in the lining of the uterus. This usually happens 6-12 days after fertilization and can be mistaken for a light period, leading to an inaccurate LMP.
Another reason for the discrepancy could be that the baby is growing at a different rate than expected. This can happen if the baby is smaller or larger than average. It can also happen if the baby is growing at a slower or faster rate than expected.
Lastly, the timing of the ultrasound can affect the gestational age. If the ultrasound is done early in the pregnancy, the gestational age may be underestimated. If the ultrasound is done later in the pregnancy, the gestational age may be overestimated.
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The discrepancy between LMP and ultrasound gestational age can have several implications. If the ultrasound due date is earlier than the LMP due date, it could mean that the baby is not as far along as originally thought. This could affect the timing of important tests and procedures, such as the glucose tolerance test and the Group B strep test. Additionally, if the ultrasound due date is later than the LMP due date, it could mean that the baby is at risk of being born prematurely.
It is difficult to say which due date is more accurate because it depends on the individual situation. If the LMP is known and the menstrual cycle is regular, the LMP due date is likely to be accurate. However, if the menstrual cycle is irregular, the LMP due date may not be accurate. Additionally, if the ultrasound is done early in the pregnancy, the ultrasound due date may not be accurate.
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It is recommended that women go by the ultrasound due date if there is a discrepancy between the LMP and ultrasound gestational age. This is because the ultrasound due date is based on the size of the baby, which is a more accurate way to determine the due date. However, if the LMP due date is more accurate, the healthcare provider may recommend going by that due date instead.
If there is a 1 week difference between LMP and ultrasound gestational age, it is usually nothing to be concerned about. This small discrepancy is within the range of normal and is unlikely to affect the timing of important tests and procedures.
If there is a 2 weeks difference between LMP and ultrasound gestational age, it is important to discuss this with the healthcare provider. This larger discrepancy could indicate that the baby is not growing at the expected rate or that there is a problem with the pregnancy. The healthcare provider may recommend additional ultrasounds or tests to monitor the pregnancy and ensure the health of the baby.
The due date of a baby is an important milestone in pregnancy, but it can also be confusing and overwhelming. It is not uncommon for there to be a discrepancy between LMP and ultrasound gestational age, and there are several potential causes for this difference. If there is a discrepancy, it is important to discuss this with the healthcare provider and follow their recommendations for monitoring the pregnancy. By working together, women and their doctors can ensure the health of the baby and a safe delivery.
1. Kullinger M, Granfors M, Kieler H, Skalkidou A. ( 2018). Discrepancy between pregnancy dating methods affects obstetric and neonatal outcomes. Sci Rep.
2. Morgan JA, Cooper DB. (2022). Pregnancy Dating. NCBI
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