How we calculate

Every date in CycleHelm comes from a published method, worked out the same way everywhere it appears. Here is each one, what it assumes, and where it falls short. Sources were read on November 1, 2026.

Ovulation and the fertile window

Ovulation is estimated as your next period's start minus your luteal phase, which is 14 days unless you tell us otherwise or your tests show something different. The fertile window is the five days before ovulation and the day itself, because sperm survive up to about five days and the egg about one (Wilcox and colleagues, 1995). The best days are the two before ovulation and the day itself.

Every estimate carries a range. With nothing but a cycle length it's plus or minus two days, the usual variation in the luteal phase. Each cycle further ahead widens it, because small differences add up.

Irregular cycles

The fertile window calculator for irregular cycles uses the calendar method: the first fertile day is your shortest cycle minus 18, the last is your longest cycle minus 11. It is deliberately wide. It is not reliable for avoiding pregnancy, and we don't present it as such.

Learning from your history (Pro)

Your cycles are the gaps between the period starts you log. A gap under 15 days or over 90 is almost always a missed or doubled log, so it's left out of the averages and we tell you. Your typical length is a weighted average of your last 12 cycles, with the most recent counting most, so a pattern that changes is followed.

The range combines how much your own cycles vary (their standard deviation) with how much your luteal phase varies. When you log ovulation tests, the day after your first positive in a cycle is taken as ovulation, and the days from there to your next period are that cycle's luteal phase. With two or more, your own luteal phase replaces the 14-day default.

We flag cycles that vary by eight days or more, and recent cycles shorter than 21 or longer than 35 days, with a note that it's worth raising with a clinician.

Due dates

From your last period: first day plus 280 days, moved by your cycle length minus 28 (Naegele's rule, cycle-adjusted).
From conception: plus 266 days.
From an IVF transfer: plus 266 days minus the embryo's age (263 for day 3, 261 for day 5, 260 for day 6).
From an ultrasound: the scan date plus 280 days minus the gestational age it measured.

ACOG notes that a first-trimester ultrasound is the most accurate method when no IVF date exists. Gestational age is counted from the dated last period: the due date minus 280 days is day 0. The first trimester runs to 13 weeks 6 days, the second to 27 weeks 6 days.

Conception dates

The most likely day is the due date minus 266 days. The window around it, five days before to one day after, covers when intercourse could have led to the pregnancy.

The pregnancy guide and visits

Baby sizes are rounded population averages, measured crown to rump until about week 20 and crown to heel after. Development notes follow ACOG, the NHS week-by-week guide and major hospital references. The visit schedule is the common US pattern described by ACOG and CDC. Your practice may differ, and your clinician's advice comes first.

What we don't do

We don't diagnose, give medical advice, or offer a method of birth control. When something in your dates is worth a conversation, we say so in plain words and suggest who to ask.