[S]eemantham is a function celebrated in the bridegrooms house by parents of bride ,when their daughter is in her first pregnancy usually 7th month before bringing her to their home for delivery relatives and friends are invited ladies put colourful glass bangles on the pregnant ladies hands bangles are also given to lady guests. [From Desitwist]There are lots of sites giving different and/or verbatim information. One said eight months and some have lots of details. Some are forums where people are asking for information because they would like to put on such a ceremony.
Our friends' parents were here from Bangalore, India and the mother began and then the sister and then other women guests followed suit.
A Tamil-Brahmin site had a very specific list of things that were needed:
Requirements for pumsavanam and seemantham:
Turmeric powder 50 gram; sandalwood powder 10 gram; kumkumam 10 gram; plantain 25;Betel nut 100 grams; betel leaf 200; plantain leaf 6; thodutha pushpam 5 meters; mango leaf bunch 10; ghee 500 grams; haaram for the Aala mokku 6 nos; couple 2; for brass kudam 1; 2kg; raw rice 2 kg; black gram 300 grams; gingilly seeds 100 grams; vraaty 10 nos; sraai thool 2 kilo; cow's milk 200 milli; scented sticks 1 pocket; camphor 1 pocket; Brass kudam 1; vasthram for brass kudam 1; Bell 1; aasana palagai or thadukku 8 nos; visiri 1; kuthu vilakku1; oil for deepam and thiri and match box; Vasthram for aalamokku paal piliya; pethy leaf bunch 1; panri mul 1; paady 50 grams well drenched in water a day before; Veena music CD 1; pancha pathra uthirini 1; elakkai; pachai kalpooram vilamichai root; krambu; 6 nos; ammi kulavi 1; sambhavanai for the kanya girl who crushed the aaalamokku; small brass sombu for punyahavachanam;
As you can see from the pictures that some of these things had been gathered.