～せいか— Perhaps Because Of

• 年のせいか、最近疲れやすい。

  Toshi no sei ka, saikin tsukareyasui.

  Maybe because of my age, I get tired easily these days.

  Noun 年 + の + せいか tentatively offers age as the possible cause.

• 緊張していたせいか、うまく話せなかった。

  Kinchou shite ita sei ka, umaku hanasenakatta.

  Perhaps because I was nervous, I couldn't speak well.

  Verb past 緊張していた + せいか suggests nervousness as an uncertain reason.

• 天気が悪いせいか、客が少ない。

  Tenki ga warui sei ka, kyaku ga sukunai.

  Maybe because the weather is bad, there are few customers.

  い-adj 悪い + せいか speculates that bad weather is behind the low turnout.

• ・Expresses tentative, uncertain reasoning — 'maybe it's because'.
• ・The result can be neutral or negative; certainty is lower than せいで.
• ・Often begins a sentence explaining an observed effect.

Commonly confused with

The JLPT rarely asks whether you recognise a pattern — it asks which of several similar patterns fits. These are the ones ～せいか is tested against.

• ～あまり(に) — So Much That It Overwhelms
• ～おかげで — Thanks To
• ～せいで — Because Of (And I Blame It)
• ～からといって — Just Because
• ～ことから — Judging From The Fact
• 次第だ／次第で — It All Depends
• だけあって — Living Up to It
• ～につれて — As One Thing Changes, So Does Another