A Faransa, ana tsammanin ranar haihuwar da aka tsara watanni tara bayan ranar da ake tsammanin fara ciki, watau makonni 41 (makonni na rashin haihuwa, watau makonni ba tare da haila ba) (1). Misali, idan kwanan wata na ƙarshen zamani shine 10 ga Maris, an kiyasta farkon ɗaukar ciki, a cikin yanayin hawan ovulatory na yau da kullun, 24 ga Maris; saboda haka an saita DPA a ranar 24 ga Disamba (Maris 24 + watanni 9). Don yin wannan lissafin, likitan mata ko ungozoma suna amfani da “diski na ciki”.

Koyaya, wannan shine kwanan wata ka'idar da abubuwa daban -daban zasu iya tasiri:

• tsawon lokacin sake zagayowar: wannan hanyar lissafin yana aiki don hawan keke na yau da kullun na kwanaki 28
• kwanan ovulation wanda zai iya bambanta, koda akan sake zagayowar yau da kullun, ko ma daga wannan sake zagayowar zuwa wani
• lokacin rayuwa na kwai da maniyyi, wanda zai iya shafar ranar hadi

Wani kayan aiki zai ba da damar tabbatarwa ko gyara wannan ranar ta farko: duban dan tayi na farko da aka yi a 12 WA kuma haka ma ake kira “Dating ultrasound”. A lokacin wannan duban dan tayi, likita zai kirga adadin 'yan tayi, ya duba kuzarinsa kuma ya yi biometry (daukar ma'aunai) wanda zai ba da damar kimanta shekarun ciki don haka ranar da ake sa ran haihuwa. Za a auna:

• tsayin cranio-caudal ko LCC, wanda yayi daidai da tsawon kai zuwa gindi na amfrayo
• da biparietal diamita ko Bip, wato diamita na kwanyar

Ana kwatanta waɗannan ƙimar guda biyu tare da lanƙwasa masu tunani kuma suna ba da damar yin ciki da kimanta shekarun tayi zuwa cikin kwanaki 3. Ana ɗaukar wannan duban dan tayi mafi kyawun hanyar saduwa da juna biyu (2).

Ko da duban dan tayi zai iya dogaro da shekarun ciki, har yanzu akwai wani bayanan da zai iya shafar ranar haihuwa: tsawon lokacin ciki da kansa. Duk da haka, wannan kuma kimantawa ce; haka ma, a ƙasashe da yawa, ba a lissafa tsawon lokacin ciki a watanni 9 amma mako guda kafin hakan, watau makonni 40. (3) Dangane da hanyoyin lissafi, abubuwan gado da wasu halaye na mahaifa, tsawon lokacin ciki yana bambanta tsakanin kwanaki 280 da 290 daga ranar farko na lokacin ƙarshe (don sake zagayowar na kwanaki 28). Don haka tsawon lokacin ciki yana bambanta tsakanin makonni 40 + 0 da 41 + 3 (4). Wani binciken da aka yi kwanan nan (5) har ma ya nuna cewa matsakaicin lokacin daga ovulation zuwa haihuwa ya kasance kwanaki 268 (watau makonni 38 da kwana 2) tare da rarrabuwar kawuna (har zuwa makonni 5) gwargwadon uwa.