Dating is from LMP. So when you are 2 weeks pregnant this is your conception day the embyo(s) are still in the fallopian tube until day 19 ( 3-5 days from conception). Implantation takes place and a yolk sac is seen at 4 weeks but hard to tell if multiple gestation sacs are there as there are many forms of pregnancy one sac 2 embryos 2 sacs 2 embryos and those with one or 2 placentas etc... I will not go into specifics here. By week 5 (3 weeks past ovulation) is the earliest fetal cardiac activity can be seen and thus 2 crown rump lengths can be detected.
Another way to know if your carrying multiples is by the quantitative beta HCG which will be doubled.
On another note up to 25% of twins will vanish by 10 weeks meaning only 1 will be viable and carried to term Thus the Vanishing twin designation
Twins are a complicated pregnancy and should be managed by a subspecialist increased risks include diabetes,preeclampsia, pre-term labor, and need for c-section to name a few.
good luck