As it happens my spatial reasoning is poorer than ever just lately, so I find the fruit analogy useful. This graphic list from Bump.com is a good example.
But check your source before you start picturing your fruity baby. For example, at week 9, an American fetus is the size of a grape. Now I know that American grapes can be enormous. An English baby at that stage would be better described as the size of an English apricot, I think. Or perhaps a little greengage.
What's more, US sites have the baby at a fig size in week 11, and a lime in week 12. In the US, figs are small - smaller than limes even. But by European fruit sizes that means a dramatically shrinking baby.
|On the left, an 11wk fetus. On the right, a 12 week fetus.|
Maybe it's safer, after all, to stick to inches and centimetres. Even for my poor pregnancy-addled mind.