waktu subur

Hi mommy nk tanye skt ye, sy biasa ovulate on cd 16 ataw cd 17 sbb cycle sy 30 ari... Tapi bulan ni sy start test opk on cd13 positive pula ,lepas tu test petang 6 pm dh negative.. Soalan sy adakah possibility ovulate on cd13 sbb sy tiap2 bulan biasa ovulate on cd16 ataw cd17? Kurang phm.. Tlg share pengalaman n share ur opinion. Tq