As there are few (less than 100) pregnant women aged under 16 and 45 or over, there may be large year-to-year changes due to natural volatility.
Gan nad oes llawer (llai na 100) o fenywod beichiog sydd o dan 16 oed neu’n 45 oed a throsodd, gallai fod newidiadau mawr o flwyddyn i flwyddyn oherwydd anwadalrwydd naturiol.