When am I most fertile?
Enter the first day of your last period and your average cycle length. We show your fertile window, ovulation day, the next period, and a due-date estimate if you conceive this cycle.
What is ovulation,
and why does the
fertile window matter?
Your body releases one egg roughly once a month. The few days surrounding that release are the only days a pregnancy can begin in a given cycle.
Knowing those days, with reasonable accuracy, is the single most useful piece of information for anyone trying to conceive, or trying to avoid it.
- 14d before next period
- 6d fertile window
- 1 egg per cycle
From day 1 to your next period.
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01
Menstrual
Day 1–5
The uterine lining sheds as a period. Hormones are at their lowest. Day 1 of bleeding is day 1 of the next cycle.
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02
Follicular
Day 6–9
Oestrogen rises, the lining rebuilds, and a follicle on one ovary prepares to release its egg.
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03
Fertile & ovulation
Day 10–16
The six-day window opens, peaks at ovulation (the LH surge), then closes once the egg is no longer viable.
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04
Luteal
Day 17–28
Progesterone rises to support a possible pregnancy. If the egg is not fertilised, hormones fall and the next period begins.
Track the body, not just the calendar. Two or three of these signs in the same week is a better predictor than any app.
Two ways to know where you are.
The calculator gives you the math, working backwards from your next predicted period. Your body gives you the confirmation. Used together, in the same week, the two reveal your fertile window with confidence.
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01
Mark day one of bleeding
The day you spot, not the day flow becomes heavy. That is day 1 of your cycle and the anchor for every other date the calculator gives you.
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02
Average your cycle length
Days from one period start to the next. Most adults sit between 21 and 35 days, with 28 as the textbook average. If yours varies, use the mean of the last three cycles.
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03
Subtract 14 to find ovulation
The luteal phase is famously stable at about 14 days. Counting back from your next predicted period lands you on ovulation, then widen by 5 days before and 1 day after for the fertile window.
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01
Cervical mucus changes
It becomes clearer, more slippery, and stretchier, similar in texture to raw egg white. The clearer and stretchier, the closer to ovulation.
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02
BBT ticks up
Basal body temperature climbs by roughly 0.3°C just after ovulation and stays raised through the luteal phase. Best measured first thing on waking.
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03
One-sided pelvic ache
Known as mittelschmerz, a brief twinge on one side of the lower abdomen as the follicle releases. Lasts minutes to a few hours.
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04
Higher libido
Oestrogen and testosterone peaks around mid-cycle nudge desire upwards for many women. Subtle but consistent month to month.
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05
Breast tenderness
Some notice fuller, slightly sore breasts as hormone levels shift through the fertile window. Usually mild and short-lived.
Before you start tracking.
01 How accurate is this ovulation calculator?
It estimates the fertile window using the standard 14-day-before-period rule. That is a guide, not a diagnosis. Cycles vary, and stress, illness or hormonal changes can shift ovulation. Pair this calculator with ovulation strips or a follicular scan for a sharper picture.
02 What if my cycle is irregular?
Enter your average cycle length and treat the window as wider than what the calculator shows. If your cycles vary by more than 7 days, book a fertility consult, a hormonal panel and a follicular scan together can find the cause.
03 Can I get pregnant outside the fertile window?
Yes, although it is less likely. Sperm can live for up to five days inside the body, ovulation can shift earlier or later than predicted, and stress changes cycles. Use this window as a planning guide, not as contraception.
04 When should I see a fertility specialist?
If you are under 35 and have been trying for 12 months, or 35-plus and trying for 6 months, book a consult. Also book sooner if your cycles are very irregular, if you have a known condition like PCOS or endometriosis, or after a pregnancy loss. The first consult at Zivah is free.
05 Is my data stored or shared?
No. The calculation runs entirely in your browser. We do not save your dates, send them anywhere, or use them for analytics.
Trying for a baby?
Let’s talk it through.
A 30-minute consult with a Zivah specialist is the fastest way to understand your cycle, your hormones, and your options. The first one is on us.
Doctor-led · In person or video · First consult free
This calculator is a planning guide based on average cycles. It is not a medical diagnosis and cannot replace a doctor’s evaluation. For personal advice, book a consult.