Fertile Days
Why there is more than one
As described in the previous article “The Period”, your body prepares in every cycle to welcome and host a very special guest.
But before that guest can arrive, two others need to meet at just the right time: a sperm and an egg.
But why a window?
After ovulation, the egg can only be fertilized for about 12–24 hours. Sperm have more staying power: under favorable conditions they can survive in the body for up to five days and wait for the egg.
That is why there are fertile days: a window around ovulation in which intercourse can lead to pregnancy.
And because ovulation cannot be predicted to the exact day, this window cannot always be planned precisely either.
The journey to the egg
A long way for tiny swimmers: discover how sperm travel through the cervix and uterus, how they can wait several days in the fallopian tube for the egg – and why the big meeting becomes possible at ovulation.
This is where sperm enter the body
After intercourse, sperm enter the vagina and begin their way upward. The acidic environment of the vagina is their first natural barrier.
- After intercourse, sperm enter the vagina and begin their way upward. The acidic environment of the vagina is their first natural barrier.
- The cervix is the gateway to the uterus. Around the fertile days, the cervical mucus becomes more permeable and makes it easier for sperm to pass.
- In the uterus the path divides: sperm swim into both fallopian tubes.
- Sperm can survive for up to five days in both fallopian tubes and wait for an egg. Which side ovulation happens on varies from cycle to cycle – sperm are often already in place on both sides.
- At ovulation, the egg enters the fallopian tube – here on the right side. There it meets the waiting sperm: fertilization can take place!
- From the entrance via the cervix to the waiting zone and the meeting point in the fallopian tube: all stops interlock in time.
Is every day equally fertile?
No. The probability of pregnancy varies within this window. It is particularly likely in the one to two days before the actual ovulation.
So the window does not mean that every day offers the same chance.
Can ovulation shift?
Yes. Even in regular cycles, ovulation can happen earlier or later.
Important: Egg-Tracker cannot detect such deviations. The fertile days it shows are therefore estimates and are not suitable for contraception.
Now we know: the fertile window lies around ovulation (day 0).
But how reliably can this day actually be predicted? And what happens to the fertile days when ovulation shifts?
On to lesson 3: OvulationScientific background
- Timing of sexual intercourse in relation to ovulationWilcox AJ et al. N Engl J Med. 1995. PMID: 7477165.
- The timing of the “fertile window” in the menstrual cycle: day specific estimates from a prospective studyWilcox AJ et al. BMJ. 2000. PMID: 11082086.
The contents are for general educational purposes and do not replace medical advice. Egg-Tracker displays estimated cycle phases, including a fertile window. This information is not a medical diagnosis and not a recommendation, and in particular must not be used for contraception (Disclaimer).