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Fertility calculator
Work out your expected ovulation and fertile window from your last period and your cycle length. You get the best days to try, and roughly when your next period is due.
Pick a date above and we will do the rest.
If you are considering becoming pregnant, it is recommended that you take 400 micrograms of folic acid daily. The Danish Health Authority recommends starting as soon as you begin planning the pregnancy and continuing until the end of week 12.
The calculation is an estimate based on your own figures. Ovulation can shift from month to month, even with a regular cycle.
How the calculator works
A cycle has two halves, and only one of them is stable. The time from ovulation to the next period – the luteal phase – lasts about 14 days for most people, while the first half varies. So we count backwards from your next expected period rather than forwards from your last one: ovulation lands about 14 days before the next bleed, whether your cycle is 24 or 34 days.
We place the fertile window from five days before ovulation to the day after. Sperm can survive up to five days in the cervix, while the egg can only be fertilised for about a day – which is why the window sits lopsided around the day itself.
If your cycle varies
If you choose “irregular”, the calculator gives a range instead of a single date. That is the more honest answer: a precise day would be a guess dressed up as a result. If you want something firmer, an ovulation test from the pharmacy catches the hormone surge a day or two before ovulation, and your basal body temperature rises slightly once ovulation has happened.
Folic acid, while you are still trying
The Danish Health Authority recommends 400 micrograms of folic acid daily from the point you start planning a pregnancy until the end of week 12. That is before the positive test, not after.
Frequently asked questions
When do I ovulate?
The time from ovulation to your next period is the most stable part of the cycle and lasts about 14 days. So we count backwards from your next expected period rather than forwards from your last one. With a 28-day cycle ovulation lands around day 14, with a 32-day cycle around day 18.
How many days a month am I fertile?
About six days. The egg can be fertilised for roughly a day after ovulation, but sperm can survive up to five days in the cervix. That is why the days leading up to ovulation count – they are the ones that produce most pregnancies.
Which days are best?
The two days before ovulation and the day itself. That is when a mature egg and sperm are both ready at the same time.
How accurate is the calculation?
It is an estimate based on your own figures. Ovulation can shift from month to month, even with a regular cycle – illness, travel, sleep and stress can move it. Treat the date as the middle of a likely range, not as an appointment in the calendar.
What if my cycle is irregular?
Then the calculator gives a range instead of a single date. If you want a firmer answer, an ovulation test from the pharmacy catches the hormone surge a day or two before ovulation, and your basal body temperature rises slightly once ovulation has happened.
When can I take a pregnancy test?
At the earliest when your period is late. Some tests can be taken a few days earlier, but the answer gets more reliable the longer you wait.
Should I take folic acid while we are trying?
Yes. The Danish Health Authority recommends 400 micrograms of folic acid daily, starting as soon as you are planning the pregnancy and continuing until the end of week 12.
Can you scan me before I am pregnant?
Yes. A fertility scan gives a picture of the ovaries and uterus and comes with or without a medical record.
When the test is positive
From week 5+0 we can see the pregnancy itself and confirm that it is in the right place. The heart starts beating around week 6.