Ovulation after childbirth & while breastfeeding

When can I expect to ovulate for the first time after giving birth? 

When the first ovulation occurs again and fertility returns after having a child varies greatly from woman to woman. When the first ovulation occurs depends primarily on whether and how regularly a woman breastfeeds her child. Women who do not breastfeed at all or only partially breastfeed usually ovulate relatively quickly and thus also have a normal cycle. Studies have shown that up to 50% of women who do not or only partially breastfeed will have a menstrual period again in the first six weeks after having a baby. For women who breastfeed for a long time, it can take several months. Complex hormonal processes play a role here, in which the milk-forming hormone prolactin is predominantly involved.
Frau die ihr Kind die Brust gibt

When can I use trackle again after giving birth / breastfeeding to determine my fertility?

As already described, there is no cycle during the breastfeeding period and it takes time until the first ovulation occurs again. When this occurs depends on the release of the hormone prolactin. During this time you have to assume that you are fertile, because this first ovulation takes place BEFORE the first menstruation has started again - i.e. unnoticed, because nobody can say exactly when it will take place.
For the evaluation (and also for forecasts for the next cycle), trackle needs a complete cycle (i.e. from the first day of menstruation to the first day of the next menstruation with an ovulation measured and recognized by trackle in between). So trackle cannot detect this very first ovulation.
Those: Natural & safe: The practice book. Safe birth control with no side effects. Family planning with sensiplan. With the right timing to the desired child (18th ed.). (2011). TRIAD.