B'Sha'ah tovah!

Even old blood can render a woman niddah. Dark red is generally a problematic color.

The size of the stain is a relevant factor in this situation. If each individual stain found on the underwear was smaller than a gris (the size of a US dime or Israeli shekel), the stains may be disregarded and did not render you niddah.

If you are unsure of the size of the stains (they may have been smaller than a gris but you don't fully recall), there is room to be lenient due to a combination of factors: you no longer have the stain to be evaluated and there is doubt regarding their size, you are pregnant and halachically considered a mesuleket damim (a woman who does not menstruate), and the stain was found externally and not on an internal bedikah.

Only if you are certain that the stain was larger than a gris, and that the color was dark red, must you assume you were rendered niddah. If this is the case, you need to wait the minimum days before performing a hefsek taharah and counting the clean days.

If you experience difficulty completing the clean days due to staining, please don't hesitate to get back to us for further guidance.

Going forward, you should be careful to wear colored underwear (or a disposable pantyliner) at all times when you are tehorah to avoid finding problematic stains. See our article on stains for more details.

Please feel free to get back to us with any further questions.

We wish you a smooth and healthy pregnancy.

B'Hatzlacha!