Pu­blished: 10. Fe­bruary 2022 | Up­dated: 14. De­cember 2023 Author: An­drea Helten | Re­viewed by An­drea Helten

Ovu­la­tion cal­cu­lator 

Cycle cal­cu­lator: How to in­crease your chances of pregnancy

If you want to have a baby, you often can’t wait for the pregnancy you’ve been lon­ging for. But un­pro­tected se­xual in­ter­course alone is often not en­ough. For fer­ti­liza­tion to ac­tually occur, the sperm must meet the fer­tile egg. An ovu­la­tion cal­cu­lator helps you to cal­cu­late the fer­tile days and thus de­ter­mine the best time for se­xual in­ter­course.

Un­der­stan­ding the cycle: When are the fer­tile days?

A wo­man’s cycle is sub­ject to na­tural fluc­tua­tions. On average, we as­sume a 28-day cycle, but it can be shorter or longer. In this average cycle, ovu­la­tion oc­curs on the 14th day. It is im­portant for pregnancy that the se­cond half of the cycle lasts long en­ough for a good ute­rine li­ning to build up in which the fer­ti­lized egg can im­plant.

The fe­male cycle is di­vided into three phases:

  • the pro­li­fe­ra­tion phase (be­fore ovu­la­tion)
  • the ovu­la­tion phase (around ovu­la­tion)
  • the lu­teal phase (after ovu­la­tion)

The ovu­la­tion ca­lendar can iden­tify the days in a wo­man’s cycle when the chances of pregnancy are par­ti­cu­larly high. Fer­tile days are the pe­riod around ovu­la­tion when an egg is ca­pable of fer­ti­liza­tion. The window for this is 12 to 18 hours after ovu­la­tion. Ho­wever, se­xual in­ter­course does not only lead to pregnancy in this small window of time. Sperm can sur­vive bet­ween three to five days in a wo­man’s body, “wai­ting” for the egg. As a re­sult, women can be fer­tile even in the days be­fore ovu­la­tion. The ovu­la­tion cal­cu­lator helps to de­ter­mine exactly this time.

Did you al­ready know? For most women, the cycle is bet­ween 21 and 35 days. If there are si­gni­fi­cant de­via­tions from this cycle length, a doctor should be con­sulted, for ex­ample, to rule out PCO syn­drome or lu­teal in­suf­fi­ci­ency.

The ovulation calculator

How does an ovu­la­tion cal­cu­lator work?

The ovu­la­tion cal­cu­lator is pro­grammed to de­ter­mine the time when un­pro­tected se­xual in­ter­course or sperm transfer into the fe­male body can lead to pregnancy. The fol­lo­wing in­for­ma­tion is needed for this pur­pose:

  • the first day of the last pe­riod
  • the average length of the cycle

Based on these data, the ovu­la­tion ca­lendar shows the ex­pected day of ovu­la­tion and the be­gin­ning and end of fer­tile days.

How re­liable are the re­sults of the ovu­la­tion ca­lendar?

If you have a very re­gular cycle, the ovu­la­tion cal­cu­lator pro­vides quite re­liable re­sults. Ho­wever, these are only in­di­ca­tions and not re­liable pre­dic­tions. For ex­ample, you cannot tell from the ovu­la­tion cal­cu­lator whe­ther ovu­la­tion is ta­king place at all, which is the most im­portant pre­re­qui­site for pregnancy. In ad­di­tion, the ovu­la­tion cal­cu­lator does not take into ac­count the na­tural fluc­tua­tions of a monthly cycle, which can be caused by stress or ill­ness for ex­ample.

How does a fer­ti­lity cal­cu­lator work for an ir­re­gular cycle?

If you have a very ir­re­gular cycle, it will be dif­fi­cult to de­ter­mine ovu­la­tion just by using an ovu­la­tion ca­lendar. To know ap­pro­xi­m­ately when your fer­tile days are, you can, cal­cu­late the sum of all cycle days from six cy­cles and di­vide the re­sult by six (the number of cy­cles). This will give you your average cycle length. Ho­wever, if you have an ir­re­gular cycle, you should al­ways use other in­di­ca­tors in ad­di­tion to the ovu­la­tion ca­lendar.

Fur­ther in­di­ca­tions for the fer­tile days

To in­crease the re­lia­bi­lity of your re­sults, you can use other in­di­ca­tors to de­ter­mine your fer­tile days and get pregnant faster. The cer­vical mucus pro­vides in­for­ma­tion about the cur­rent cycle. If it draws th­reads like raw egg white, then the fer­ti­lity window is most li­kely opened. Pal­pa­ting the cervix and me­a­su­ring the tem­pe­ra­ture also helps to de­tect ovu­la­tion.

When is a pregnancy test wort­hwhile?

About seven days after fer­ti­liza­tion, im­plan­ta­tion oc­curs and the fer­ti­lized egg be­gins pro­du­cing the pregnancy hor­mone HCG. From the eighth day after im­plan­ta­tion, the hor­mone can be de­tected in the urine. Pregnancy tests now al­ready show the cor­rect re­sult with a pro­ba­bi­lity of over 90%.


Some cou­ples use the ovu­la­tion cal­cu­lator as part of na­tural con­tracep­tion. The ovu­la­tion ca­lendar can be used to de­ter­mine the fer­tile days when ad­di­tional con­tracep­tives such as con­doms would need to be used to pre­vent pregnancy.

Ho­wever, the ovu­la­tion ca­lendar is only con­di­tio­nally sui­table for con­tracep­tion and in no case as the sole mean of con­tracep­tion, as it does not pro­vide 100% re­liable re­sults. If the cycle shifts for ex­ample, pregnancy can still occur.

Which is the best ovu­la­tion ca­lendar?

On­line you can find many cycle cal­cu­la­tors, which are all based on the same me­thod. For ex­ample, the ovu­la­tion cal­cu­lator from Ba­belli shows you when the chances of pregnancy are good. In the end, you can see for yourself which ca­lendar you like best.

About Fer­tilly

At Fer­tilly, we have made it our mis­sion to ac­com­pany cou­ples (ho­mo­se­xual and he­te­ro­se­xual) and sin­gles on the way to ful­fil­ling their child wish. In doing so, it is im­portant to us to create trans­pa­rency in the area of fer­ti­lity ser­vices, to pro­vide in­for­ma­tion and know­ledge on the to­pics of pregnancy and fer­ti­lity and to help you to find the most sui­table Fer­ti­lity Center. Th­rough co­ope­ra­tion with first-class Fer­ti­lity Cen­tres and cli­nics in Eu­rope, en­qui­ries about Fer­tilly are given pre­fe­ren­tial tre­at­ment. This means that our pa­ti­ents avoid the usually long wai­ting times and get ap­point­ments more quickly.

If you would like more in­for­ma­tion about Fer­ti­lity Cen­ters, suc­cess rates and prices, please contact us using this ques­ti­on­n­aire. We will ad­vise you free of charge and wi­t­hout any ob­li­ga­tion.

  • Answer the first ques­tions in the on­line form in order to book an ap­point­ment. This way we can better ad­dress your needs du­ring the con­ver­sa­tion.

  • We will find the best contact person for your in­di­vi­dual needs. Sche­dule 20 mi­nutes for the con­sul­ta­tion.

  • We will in­tro­duce you to the right fer­ti­lity clinic from our net­work, make an ap­point­ment and ac­com­pany you until your wish for a child is ful­filled.

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