Conception Question

[gtk25]

How do you figure out when conception/ fertilization occurred based on a due date?  And can the due date be inaccurate in an ultrasound performed at 6 weeks, 1 day?

[AnnieBrooke]

I'd trust an ultrasound performed that early.  When I did IVF and so knew my exact conception point, the ultrasound tech also nailed it to the day, when I was 7 weeks 1 day.  The crown-to-rump measurement is consistent among all babies that early in the pregnancy, and as the above poster said, it will vary later due to different growth rates in the embryo.  But an early ultrasound is quite reliable.  I would not trust it to the day, but I would trust it to be correct within three days.  

If you have a due date and if you have the date at which the ultrasound tech told you 6w1d, you can go online to a conception calculator and it will give you an answer to your question.  But again, with the 6w1d ultrasound, you can take an educated guess that conception was around 4 weeks and 1 day prior to the ultrasound.  This supposes that you ovulate right at mid-cycle in a 28-day cycle.  If your cycles are longer or shorter, you would need to adjust slightly and say maybe three weeks and 6 days before the ultrasound, or four weeks and two days before it.

[bluegenie2003]

Ultrasounds that are performed that early are usually pretty accurate since the baby changes so much every day at that point. If you were given a age of 6 weeks 1 day, that means that conception occured AROUND 4 weeks 1 day before the ultrasound was performed (they add two weeks to the conception date since gestational age goes by the date of your last period). Its really almost impossible to pinpoint the exact day unless you had fertility procedures done however.