American Kristin Armstrong capped her return to the sport following the birth of her baby by successfully defending the women's Olympic time trial crown on Wednesday.

The American, who returned to cycling after the birth of her baby nearly two years ago in a deliberate bid to defend her crown, then announced she'd taken part in her final competitive race.

"I am now officially retired," said the 38-year-old Armstrong, who won by a convincing 16 seconds from world time trial champion Judith Arndt of Germany.

Armstrong clocked a winning time of 37min 34sec for the 29 km race against the clock around London's Hampton Court Palace.

Arndt took the silver in 37:50 while Zabelinskaya, already the winner of a road race bronze on Sunday, took her second medal of the Games in a time of 37:57.

Danish-born New Zealander Linda Villumsen finished just two seconds off the bronze medal in fourth with Canadian Clara Hughes in fifth.

Emotional win

Armstrong, no relation to seven-times Tour de France champion Lance Armstrong, fought back the tears on the podium as she received her gold medal and then cradled her son in her arms.

Despite giving birth fewer than two years ago and virtually retiring from the sport, she said the lure of another Olympic gold was just too strong.

But she said this was definitely her final race.

"When I came back, everyone asked me why in the world would I come back," said the American.

"And the reason I came back was because the feeling I got in Beijing, nothing could top that, but I couldn't imagine being on the top step of the podium with my son, Lucas, in my arms."

Starting last from the field Armstrong, riding with a sore elbow following a crash in the Olympic road race, kept the pace high throughout and came through both intermediate points with an advantage over her rivals.

But despite showing signs of celebration just before the finish line, she claimed she had been kept in the dark about the result.

"Today I didn't know (I had won) until I crossed the finish line. People try to tell you you are on track, but out on the course today, the information I was getting was that it was a close race," she explained.

"I just needed to give it everything if I really wanted it. I had my doubts a couple of days ago (after crashing in the road race on Sunday).

"There were torrential downpours, but I held it together, just like in Beijing."