If a baby is measuring week bigger on scan than EDC does that mean the big or do they date by your last period?

It depends on the situation and the individual baby's growth patterns. There are a few factors to consider when determining whether a baby is measuring big on a scan compared to the estimated due date (EDC).

1. Measurement Accuracy: Ultrasound scans are not always 100% accurate in determining the baby's size and gestational age. There can be some variability and measurement errors, especially in early pregnancy. So, while a baby might appear to be measuring a week bigger on a scan, it's possible that the measurement itself may have some uncertainty.

2. Individual Growth Patterns: Babies grow at different rates and have their own unique growth curves. Some babies may naturally be larger than others at the same gestational age. Factors like genetics, nutrition, and maternal health can influence fetal growth. A larger baby might not necessarily indicate a dating issue, as long as the growth is consistent with the baby's own pattern.

3. Last Menstrual Period (LMP) Accuracy: The EDC is primarily calculated based on the last menstrual period (LMP) reported by the mother. If the LMP date is not accurate or there was uncertainty about the exact date of ovulation, it could lead to a slight difference between the EDC and the actual gestational age.

4. Scan Timing: The timing of the scan can also affect the measurements. If the scan is performed too early or too late in the pregnancy compared to the estimated gestational age, it may result in variations in the measurements.

5. Clinical Assessment: In cases where there is a significant difference between the scan measurements and the EDC calculated from the LMP, doctors or healthcare providers will consider other factors like the mother's menstrual cycle history, physical examination, and any previous scan results to assess the overall growth and development of the baby.

It's essential to have regular prenatal care and consultations with your healthcare provider to monitor your pregnancy progress and ensure the well-being of your baby. If there are any concerns about the baby's size or gestational age discrepancies, your provider will discuss the findings with you and recommend appropriate actions, including additional scans or tests if necessary.