In cases where the date of conception is known precisely, such as with in vitro fertilization, the EDD is calculated by adding 266 days to the date of conception. American College of Obstetricians and Gynecologists.
For women seeking a medical abortion procedure, a sonogram is recommended to determine if the pregnancy is viable (uterine, non-ectopic pregnancy) and for accurate pregnancy dating.
Medication Abortion – Facts and information for healthcare professionals
When trying to get pregnant, a false positive prgnancy test can be incredibly disheartening.
However, conception doesn't actually occur until around 14 days after your LMP, or later if your cycle is longer than 28 days. Multiple pregnancy: antenatal care for twin and triplet pregnancies CG129.
You may know when your LMP was, but not when you actually conceived.