Dating by ultrasound
These scans are only used to estimate the due date of your baby if this is all you have to go on.
If you have more than one ultrasound during your pregnancy, giving you 'multiple dates', then the earliest ultrasound estimate should be used, because it will be more accurate.
The dates begin on the 1st day of your last period, not the date of conception. But, if your cycle is a day or 2 more than the average of 28 days, that makes sense.
Most women ovulate approximately 14 days before the first day of their next menses. Read more For menstrual dating assuming a 28 day cycle, ovulation would have occurred approximately feb 26. Read more I think I would be more concerned that a 41yo is almost 3 weeks post dates.
The size of the baby correlates less and less with its age as time goes on.
Ultrasound examinations 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).
However, the accuracy of the ultrasound examination is always dependent on the skill of the sonographer and the quality of the equipment.
If the pregnancy is very early the gestation sac and fetus will not be big enough to see, so the transvaginal approach will give better pictures.
5 ½ weeks gestation means 5 ½ weeks from the first day of the last menstrual period, which is usually about 3 ½ weeks from the date of conception (confusing, isn’t it!
) the fetus is clearly seen on trans-vaginal ultrasound and the heart beat can be seen at this early stage (90 to 110 beats per minute under 6 to 7 weeks, then 110 to 200 beats per minute as the baby matures).
Sometimes a trans-vaginal examination shows that your pregnancy is less advanced than expected.
The earlier the ultrasound is done, the more accurate it is at estimating the baby's due date.