HI SIMT,
All supplements must give a pregnancy and breastfeeding warning if there may be a caution, even if slight.
"Do not use Chlorella if you are pregnant or breastfeeding.
Although unlikely, chlorella may cause allergic reactions such as breathing problems or tightness in your throat or chest, chest pain, skin hives, nausea, diarrhea or stomach cramps, rash, or itchy or swollen skin and heightened sensitivity to the sun. In the event of these occuring stop taking the capsules. As with all allergic reactions, it is always prudent to seek medical advice from your doctor.
Chlorella supplements can be rich in vitamin K, which can reduce the effectiveness of the blood-thinning drug warfarin."*
The advice you read may be because if you take it for one year prior to becoming pregnant you will almost be 100% sure that you are not allergic to it.
*
http://www.herbwisdom.com/herb-chlorella.htmlAll the best,
Dalene