4 ways to work out your Window of Optimum Fertility

This is a subject that people TTC know well.  But how well?  It’s always worth a quick reminder on when the most fertile time of the month is and to remember that it is different for everyone because ovulation doesn’t happen on the same day every month.

The most important thing to do is know your menstrual cycle well and to understand what the signs of ovulation are for you, and again this can be different for everyone.  Once you have these signs you can then look out for them in the following months because by the time you are ovulating, if you have not had sex, you have already lost some of your fertile days, and your fertile window is nearly over.

Sperm have the capacity to wait in the fallopian tubes for the newly released egg for a couple of days.  That’s why the most important time for sex is the 2-5 days preceding ovulation, that’s why a little bit of research on your own body will help you predict your unique fertile window.  So here are 4 ways you can do this

  1. Ovulation Predictor Kits (OPK)
    Shop bought Ovulation Predictor Kits are one way. They track a rise in the luteinising hormone (LH) which rises just before you ovulate. Easy to use, you wee on a stick to get the reading, some more sophisticated kits also track oestrogen levels which build just before LH is released.  These can be very useful but for some women with oestrogen dominant conditions e.g. poly cystic ovaries (PCO) they can be unhelpful and frustrating to use, so they are not for everyone.
  1. Basal Body Temperature charting

Basal body temperature is a simple, useful, and widely used method.  Charting temperatures over a couple of cycles can help predict your most fertile window especially when used in combination with some of the methods listed here, because it confirms, but does not predict ovulation.  BBT is taken when your body is fully at rest, so it’s taken on waking, literally the first thing you do using a digital thermometer orally for accuracy.  Then you record the temperatures on your chart or in an app.  In the follicular phase, oestrogen is dominant and has a suppressant effect maintaining body temperature at a lower level.  A day or two after ovulation the first high temperature occurs, as the corpus luteum releases progesterone which has a the effect of increasing temperature by about 0.2C.  More sophisticated kits that monitor fertility by inserting a sensor into the vagina at night with readings being downloaded to an app are also available.

  1. Cervical Mucous

Cervical mucous changes through the month, these secretions are produced in the cervix and made up of 90-98% water.  The consistency change is bought about by stringy molecules called mucins which respond to increasing oestrodiol levels.

Post period your vulva area feels dry and you may not even be aware of the mucous which is thick, pasty and impenetrable to sperm.  As oestrogen levels rise during the follicular phase, the mucous becomes more liquid and wet, even sticky – its role is to catch some of the abnormal/poor quality sperm (i.e. slow swimmers) before they reach the uterus.

The role of the stretchy fertile mucus which arrives just before ovulation is to provide a rapid transportation of the sperm to the uterus.  To facilitate this the mucins are lined up longitudinally and appear as pine needles under a microscope.  Spinn mucous is very stretchy and slippery like egg white, it is stretchy if placed between the index and thumb.

Finally the mucus makes another change which very often is not distinguished from Spinn mucous and is the most fertile of all giving the sperm a final push to pass through the cervix into the uterus.  It is not as thick and stretchy as Spinn but it is extremely lubricative and feels slippery in the vulva.

  1. Cervical position

You probably know the cervix sits at the top of your vagina – I think of it as a sort of drawbridge to the uterus – sometimes it lets stuff in, sometimes it doesn’t.  Getting in touch, literally, with your cervix and observing the changes in shape and position can provide you with some really good information about when you ovulate.  Teamed up with the BBT chart and cervical mucous your knowledge gets even better.

Most of the month you should be able to feel your cervix by inserting your finger into your vagina.  It feels like a firm round dimple and if you are not close to ovulation it is easy to find, but when you get nearer to ovulation the shape, position and texture of the cervix changes.  This is because as the hormone oestrogen begins to peak, the ligaments that support the uterus become tighter pulling the cervix higher up the vagina.  It is then a little bit more difficult to feel and some women need to squat to reach it.  Around ovulation the cervix now feels softer in texture.  As the cervical mucous is released, the a small opening in the centre of the cervix called the oz begins to open which can also be felt when palpating the cervix.

The fertile window starts at the first signs of the cervix moving higher, becoming softer and shorter in length.  Once the progesterone kicks in post ovulation, the cervix moves back to its low, firm, closed position and the fertile window is now closed.


So ovulation happens when the CM is stretchy, there is a shift in BBT, the cervix becomes higher in the vagina, and LH is released.  The days before this are the optimal time for sex.  Journalising these findings will help you discover yours and for many engenders a feeling of bringing back a degree of control.