Most advice about trying for a baby comes down to one instruction, which is to find your ovulation day and aim at it. That advice all but guarantees each month feels like a test you can fail, and it isn’t how the biology works.
Pregnancy is possible across a span of about six days, and the sign most people track hardest only confirms ovulation after it has already happened. This guide covers what to record, what each fertility sign can and can’t tell you, and the point at which to stop tracking harder and see someone.
The fertile window is six days, not one
The window exists because sperm outlive the egg by several days. ACOG gives the numbers in its guidance on fertility awareness based methods. An egg survives in the fallopian tube for about 24 hours after ovulation, while sperm can survive inside the body for about 3 days and sometimes up to 5. Pregnancy is possible from about 5 days before ovulation until 1 day after it.
The chance of pregnancy is highest when live sperm are already present at ovulation, so the days leading up to it matter more than the day itself. ACOG’s own suggestion for promoting pregnancy is intercourse every day or every other day across the fertile stretch. The NHS guidance on trying to get pregnant takes the same view, and puts preparation and regular sex ahead of precise timing.
You can keep a record that helps you cover a six-day span. No record can pin one correct day in advance, and chasing that day is how people end up with three apps disagreeing about the same cycle.
What each sign tells you
People trying to conceive track four things. The main difference between them is whether they predict ovulation or confirm it afterwards.
- Your period start dates. These are the least glamorous sign and the most useful one. ACOG notes that the number of days between ovulation and the start of the next period is the most consistent part of the cycle, so apps estimate ovulation by counting back from the next expected period. That estimate is only as good as your record of start dates. Our ovulation and period calculator runs the same backwards count from your last start date.
- Cervical mucus. This one predicts, and it costs nothing. ACOG says that just before ovulation the amount noticeably increases and the mucus becomes thin and slippery. Afterwards it decreases and thickens.
- Basal body temperature. This one only confirms. ACOG says it shows when ovulation has already occurred, not when it’s going to occur. The rise is small, roughly 0.5 to 1 degree Fahrenheit, and holds until the end of the cycle. It’s useful for answering whether you ovulated at all, and useless for planning this week.
- Ovulation predictor kits. These predict, and they’re the only sign that costs money every cycle. They detect the hormone surge that comes before ovulation, which typically buys a day or two of notice.
Start with the first two. Add temperature if you want confirmation, and kits if your cycles are irregular enough that a backwards estimate keeps missing.
Why irregular cycles change the plan
Calendar estimates assume the last few cycles predict the next one. When cycle length swings by a week or more that assumption fails, and an app can’t fix it by being more confident. ACOG notes that the Standard Days approach works best when cycles run consistently between 26 and 32 days.
If yours don’t, the body signs you can check in real time carry more weight than the calendar, because they respond to the cycle you’re in. Irregular cycles are also worth looking into for their own sake, since they can point to something treatable. Our guides on why periods go irregular and tracking with PCOS cover the common causes, and how the fertile window is calculated explains where the estimate comes from and where it breaks down.
When two of you are trying
Trying to conceive is usually a two-person project tracked by one person, which is how the mental load lands entirely on whoever owns the app. Sharing the dates is the cheapest fix available, and it doesn’t mean sharing a diary. Knowing which week the two of you are in is enough to plan around, and it removes the recurring conversation that starts with someone asking where things stand.
Our guide on sharing your cycle with a partner covers what to share and what to leave out.
When tracking is no longer the answer
Tracking is a way to use the months well. It isn’t a treatment, and ACOG names the point at which more careful tracking stops helping.
ACOG’s guidance on evaluating infertility recommends an evaluation if you haven’t gotten pregnant after 1 year of regular sex without birth control. If you’re older than 35 that drops to 6 months, and if you’re older than 40 the advice is to talk to an obstetrician gynecologist now instead of waiting out a trial period. In many cases the evaluation finds a treatable cause, and in many others treatment works even when it finds none.
Some signs are worth raising sooner instead of tracking through them. Cycles that keep varying by more than a week, no temperature rise across several cycles, periods that stop, or a known condition such as endometriosis or a thyroid problem all justify an earlier conversation. The NHS also lists preparation steps that have nothing to do with timing. They include folic acid and vitamin D, checking that cervical screening and the MMR vaccine are up to date, and reviewing any prescribed medicine with a GP before you start trying instead of stopping it yourself.
When you do go, bring your record. The last several start dates, your cycle lengths, and whether you saw a temperature rise answer in ten seconds what would otherwise take ten minutes of guessing.
How Plum fits in
We build Plum, a period and ovulation tracker for iPhone, so we’ll say what it does about this and skip the pitch.
Plum shows the fertile window as a range of days. It predicts from a rolling median of your recent cycles instead of a plain average, so one unusual month doesn’t drag every prediction after it. Once your cycles vary enough that a confident single date would be dishonest, it widens the window. Logging, the fertile window, and ovulation are all available without a subscription.
It doesn’t diagnose anything or tell you whether a month worked. It doesn’t replace an ovulation kit either, and it isn’t a fertility treatment. If you share with a group, they see a summary of your dates. They don’t see your symptoms or your moods.
Plum shows no ads and never sells personal data. It does require an account and syncs to our servers, encrypted in transit and at rest, so we won’t claim it stays on your phone. You can delete your account and everything in it from inside the app at any time.
