Geekmom answered it perfectly
That's not really how it works. A measurement of fundal height (from the pubic bone to the fundus) in centimeters often corresponds to gestational age for a period of time during pregnancy... but that's not an accurate way to determine what the gestational age is. The best way to do that is by counting from LMP or by early ultrasound dating. Fundal height is measured generally just to make sure that baby is on track for growth during the second half of pregnancy - it can signal a potential problem with low or excessive amniotic fluid, too - but it's not a dating tool. Your OB or midwife should certainly have a fairly good estimation of your due date long before a fundal height can be measured.