OPK-to-Ovulation Timing Calculator

What this estimates

This tool estimates the likely ovulation window after the first positive urinary LH (OPK) test, suggests the best days to try, and projects the earliest reliable home pregnancy test day and an EDD range if conceived. It combines guideline statements about the relationship between the LH surge and ovulation with simple, user-selected modifiers (reported surge length; presence/absence of a basal temperature shift). Educational use only; not diagnostic.

LH surge → ovulation window

Professional guidance indicates that ovulation typically occurs within about 1–2 days after the onset of the urinary LH surge. We center the window roughly 30 hours after the reported first positive and allow a span that narrows if a sustained basal temperature rise is reported (post-ovulation sign). Sources: ASRM Practice Committee, “Optimizing natural fertility” (2022); day-specific fertility evidence in Wilcox et al., NEJM 1995.

Intercourse timing (“best days to try”)

We highlight the OPK positive day and the following ~2 days as the most practical targets, mapping to the peak fertile window identified in prospective studies. Source: Wilcox et al., NEJM 1995.

BBT shift logic

A sustained basal body temperature rise is a retrospective marker that ovulation has likely occurred (progesterone effect). If the user reports a clear shift, the calculator narrows the ovulation window accordingly. Source: ASRM “Optimizing natural fertility” (2022).

Earliest reliable home pregnancy test day

Earliest hCG rise depends on implantation timing, which varies (commonly ~8–10 days post-ovulation). To reduce false negatives, we suggest testing at ~10 DPO from the early bound of the ovulation window and repeating in 48 hours if negative. Source: implantation timing and early loss distribution in Wilcox et al., NEJM 1999.

EDD range if conceived

Obstetric EDD is computed as ovulation (≈ conception) + 266 days, returned as a date range based on the ovulation window. Reference: ACOG patient guidance on due-date calculation (ACOG: How your due date is estimated).

Handling variable surge length

Surge duration can vary (short ~12 h; typical ~24–36 h; occasionally longer). The tool lets users select a pattern and adjusts the center/width of the window without exceeding biologically plausible bounds (roughly 12–48 h from first positive in most cycles). Guidance overview: ASRM 2022.

Limits

Urinary LH kits detect the surge, but surge onset/peak timing and user sampling frequency (e.g., first daily vs twice daily) can shift the apparent interval to ovulation. Conditions such as PCOS or very short/long surges can reduce precision. The calculator provides reasonable windows, not exact timestamps.