More than half the whales killed by Japanese whalers in the Antarctic last summer were pregnant females, the Humane Society International (HSI) said.
The group said that of the 505 Antarctic minke whales killed, 262 of them were pregnant females, while one of the three giant fin whales killed was also pregnant.
The findings came from a review of Japanese reports from their most recent 2006-07 whale hunt in Antarctic waters and were released ahead of the resumption of a Federal Court case the HSI is taking against Japanese whaling company Kyodo Senpaku Kaisha Ltd.
Japanese whalers take hundreds of whales each year in Antarctic waters for so-called scientific study purposes.
"These are gruesome statistics that the Japanese government dresses up as science", HSI spokeswoman Nicola Beynon said in a statement.
She said the HSI was hoping the court would set a date for a full hearing during Tuesday's directions hearing.
"The full hearing will be to determine whether Japanese whalers are in breach of Australian law when they hunt whales in the Australian Whale Sanctuary in Antarctica and whether the court will issue an injunction for the hunt to be stopped," Ms Beynon said.
"HSI will ask for the final hearing to be held as soon as possible before the hunt starts up again this summer.
"It has been three years since HSI launched the case and many hurdles have been overcome to get to this point.
"It horrifies Australians to know that pregnant humpback whales breeding in the warm waters off Australia this winter will be targeted by the Japanese hunters in Antarctic waters this Christmas."