Isotretinoin/Accutane Birth defects beyond end of treatment?

I took Isotretinoin for 2 weeks and stopped 10 weeks ago. However, we have found out that I am now 7.5 weeks pregnant. I had been advised by NHS that you should wait 4 weeks before getting pregnant to avoid birth defects. As I had only finished for between 2-3 weeks, what is the likelihood that there will be birth defects/complications? And what tests are available to identify defects during pregnancy?

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