Pop Quiz: Does an Ovulation Test Strip Tell You When Your Fertile Window Opens or Closes? Hint: Neither.

Karine Sabourin
Jul 29
2 min read

If you said “opens” — not quite.

If you said “closes” — also not quite.

If you said “um, isn’t that the whole point?” — girl, same.

Let’s talk about what OPKs actually tell you, and why they don’t give you the full story when it comes to getting pregnant.

What OPKs Are Really Testing

Ovulation predictor kits (OPKs) detect a surge in luteinizing hormone (LH) — a hormonal spike that typically happens 24 to 36 hours before ovulation.

So if you get a positive OPK, that means your body is gearing up to release an egg soon.

Sounds helpful, right?

But here’s the problem: by the time your OPK turns positive, you’re already deep into your fertile window — and that window might even be closing.

Let’s Talk Fertile Windows

Your fertile window actually opens before that LH surge — when estrogen starts rising and your body begins producing fertile cervical fluid.

This fluid:

Creates the perfect environment for sperm
Helps sperm survive for up to 5 days
Tells you your body is getting ready to ovulate

In other words, you can be fertile even if your OPK is still negative.

What OPKs Can’t Tell You

When your fertile window opens
Whether ovulation actually happens
If your LH surge is strong enough to be detected
Whether you’re dealing with anovulation or just a quiet cycle

They’re helpful — but they’re showing up fashionably late to the fertility party.

Let’s Layer the Tools

Instead of relying on OPKs alone, here’s the smarter approach:

Track cervical fluid to spot the start of your fertile window
Use OPKs to confirm that ovulation is likely approaching
Use BBT (basal body temperature) to confirm that ovulation actually happened