LOS ANGELES — Billionaire real estate mogul Donald Bren testified Thursday in the child support case brought by his two adult children, saying he was shocked when his then-lover told him decades ago that she was pregnant with their first child.

Bren, one of the nation’s richest men, has an estimated net worth of $12 billion and has spent a lifetime protecting his privacy.

Twenty-two-year-old Christie Bren and 18-year-old David Bren have sued him for $400,000 a month in child support retroactive to the time they were born. That comes to about $100 million.

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The 78-year-old Irvine Co. chairman told a courtroom that he suggested to Jennifer McKay Gold that they create a legal agreement to provide for child care. He said they never talked about marriage or having a family.

Four contracts were created involving child support entered into by Gold each time she became pregnant and after the children were born. The accords, beginning in 1988, rose from $3,500 a month to $18,000 a month between 1992 and 2002.

Bren recounted that he and Gold dated in the mid-1980s but she never slept over at his home and the two would see each less frequently as the years passed.

Bren said he was surprised that Gold became pregnant twice because he was under the impression she was using birth control.

“I felt I was betrayed in that she promised me she would be protected and she wasn’t,” he said while under questioning by his lawyer, John Quinn.

Bren said he saw both children after they were born a handful of times and sent them toys. He also said he paid for their college expenses.

“I felt an education at the university level, at the graduate level is perhaps the best gift a parent can give a child,” he said.

Gold gave an entirely different account of the pair’s 13-year relationship, saying she and Bren loved one another and saw each other regularly. She portrayed Bren as a jilted lover when she said she broke off the courtship in 1997 because she wanted a full-time partner.

“He was not very happy about that,” Gold told jurors. “He said I would pay for that. He said he would punish me.”

On cross-examination, however, Gold testified Bren never withheld child support payments from her after the split. She also confirmed that she received about $3 million in child support from Bren between 1988 and 2002. The payments were her only source of income for a majority of that time, she said.

While her children had many amenities growing up, Gold attempted to tell jurors why she filed the lawsuit against Bren in 2003 on behalf of her kids.

“I would have liked my children to have more in the lifestyle as their father,” said Gold, adding that Bren wasn’t keeping his promise to be involved in the kids’ lives.

Earlier, the children’s attorney Hillel Chodos pointed out to Bren that none of the legal agreements limited the amount of support payments the billionaire could make. Bren noted that Gold could have sought to increase the child support through a court order.

“She always had the right to appeal to the court,” he said.

But Gold said Bren didn’t want her to seek additional child support because of the undue attention he would receive.

“He wanted to preserve his privacy,” she said. “He was adamant about that.”

Chodos portrayed Bren during his opening statement Wednesday as a high-living executive who has two California homes, a New York apartment, a Sun Valley ranch, two yachts and five private jets.

Gold said Bren told her he spent $3 million to $5 million a month on personal expenses.

Bren, wearing a black blazer, gray pants and a powder blue shirt, remained composed during his testimony and rarely elaborated on his answers. He could be recalled to testify on Monday.