Image copyright Getty Images Image caption At the table: Wendy Sherman (far left) and Helga Schmid (far right) at a negotiation session in Switzerland

President Barack Obama continues to push for Congress to sign off on a nuclear deal with Iran. But the historic negotiation was brokered with the help of an unprecedented number of female diplomats.

When an agreement is struck among parties, it is standard practice to "shake on it" in order to seal the deal.

But when a historic nuclear accord was reached in Vienna, Austria, on 14 July between Iran and the P5+1, the Iranian negotiators could not shake the hands of their female interlocutors due to the country's strict religious customs.

Three Western women - two European and one American - were central to a comprehensive deal reached to curb Iran's nuclear programme.

Image copyright Getty Images

Leading the political side of the negotiations was Federica Mogherini, the European Union's chief diplomat and former Italian foreign minister.

She took over the post in November 2014 from Baroness Catherine Ashton - who has also been cited for playing a key role by building good relations with Iran.

Ms Mogherini had faced criticism when she first started as chief diplomat - many said she was too young and inexperienced.

But by crunch time, she had earned herself a reputation as a tough negotiator.

Alongside her was her deputy, Helga Schmid. Credited with having immense technical knowledge, her colleagues have called her a "linchpin" in the negotiations and "the ringleader".

Image copyright Getty Images Image caption Helga Schmid and Iran's Abbas Araghchi take part in the P5+1 powers and Iran talks in Vienna

In the end it was Ms Schmid who led negotiations of the agreement and its five annexes, senior Western diplomats have said.

Ms Schmid - unlike Ms Mogherini - had been well known on the diplomatic scene, with years of experience in Iran negotiations under her belt.

At the table for the Americans was Wendy Sherman, her country's first female undersecretary of political affairs and a veteran of nuclear negotiations.

She was one of the chief architects of the Clinton administration's North Korea nuclear policy and had taken the lead on the US team's nuclear talks with Iran since 2011.

Image copyright Pool Image caption Wendy Sherman says handshakes weren't needed to get an agreement

Having three women as senior negotiators in the Iran talks was unprecedented.

"Even inside the European Union it's not that often it happens that there are more women sitting at the table than men," Ms Mogherini told the BBC.

"So it was somehow new, but it's my personal feeling that it was helpful."

She says when the men veered off course and went on historical tangents or started to get tangled into debates about who gave more, the women walked them back to the present.

"The fact of having many women at the table in key positions helped us be concrete and pragmatic the whole way."

But the Islamic Republic of Iran has strict rules about interactions between men and women - customs that diplomats follow regardless of whether or not they're in the country.

"We couldn't shake hands, couldn't touch in any way at all," Sherman said.

But this practice was not uncommon to her.

"I grew up in Baltimore where there's a large Orthodox Jewish community where the same is true, but I think we all understood how to speak to each other without shaking hands and understanding each other to the extent that we got an agreement."

And she says being a woman was not a barrier in her progress in the negotiating room.

Image copyright Getty Images Image caption Catherine Ashton began the negotiations (with Schmid on her left)

"When I sat across from the Iranians I was the United States of America and perhaps as I woman I can say some things that don't come across as tough, but when I do get tough and when I do lose it, it makes a big impression because it's unexpected."

Over the course of the years she got to know her Iranian counterparts rather well and they would often share videos and stories of their families.

And Ms Mogherini said she heard doubts from her own region.

"Parts of the Western or European public have been reacting and doubting that women were able or could be respected for playing a bigger role," she said

"That says a lot about the fact that in our own societies sometimes we have to still work to pass the message that women can be top negotiators and be in top political position in a credible way as a men are in some cases even more."

Despite increased female leadership, not all teams had a female presence. Sherman said she would tease her colleagues about it.

Image copyright Getty Images Image caption Federica Mogherini speaks during a post-deal trip to Iran

"Some of the teams were all men and I hope that will change," she said.

Asked for more details she remained a diplomat.

"They know who they are, they know that they've heard from me and that they need to change their act."

Now all the talk is on how and when the deal will be implemented - which began with a highly anticipated trip to Tehran by Ms Mogherini and Ms Schmid.

The women who brought the nuclear deal to fruition have their work cut out for them - can they put it into motion?