Thanks, Ben. I'll now go into some further details on the trends we saw in Q4 as well as to provide a preliminary outlook for Q1. We'll start with a quick summary of the headlines and then go into more detail.
As Ben mentioned, we had a very strong fourth quarter. Revenue growing 76% year-over-year with adjusted EBITDA margins of 42%. Monthly active user growth remains strong with all major regions once again growing into double digits. Ben mentioned this earlier, but I want to echo it as well, because it's a tremendous achievement. We added over 100 million monthly active users globally in 2020. That's more than we've ever added in a year. It's incredibly exciting to see so many people searching for and finding inspiration on Pinterest.
Looking ahead, we expect our revenue to grow in the low 17% range year-over-year in the first quarter.
We also expect to maintain our total non-GAAP operating expenses in the first quarter at a similar level compared to the fourth quarter. There were two primary drivers to strength in Q4.
First, and similar to the third quarter, our strategy is working and our team is executing well. The investments we've made in technology, and then sales coverage are continuing to pay off.
Over the last year, we've invested in our ability to better deliver returns accountable performance advertising, including scaling, conversion optimization, ads, shopping, ads, and building improved automation to help advertisers of all sizes more easily onboard and realize the value of being on Pinterest.
We also expanded our sales team in Western Europe to monetize our engagement there. Drilling down, auto bid was once again a meaningful contributor to our strength in Q4 and was especially so for small, small and medium sized businesses. And I'm pleased that our international business grew 145% year-over-year on the back of strong advertiser demand. International markets now represent 17% of total revenue.
Given this momentum, we plan to continue to invest to best serve pinners, merchants and advertisers against the key priorities that Ben mentioned earlier. Stuck beyond our own investments, a strong overall online holiday season drove healthy advertising demand. We saw broad based strength across advertiser verticals, sizes and objectives, with the biggest improvement and growth from retail advertisers.
Importantly, our team executed well against this backdrop.
Our sales and marketing teams built an insights led go-to-market program over the course of 2020 that helped us deliver against Q4 seasonal moments. This program showcased the commercial intent of our users and helped businesses create better performing campaigns through the holiday season as a result. That helped prepare advertisers for the earlier start to holiday shopping this year, and continue seeing results as the shopping season persisted longer this year.
Let me give two examples. The Lego Company created a holiday campaign based on popular search terms on Pinterest. Seeing increasing search trends for creative kid’s gifts allowed the Lego Company to tailor and serve ads at the right moment ahead of the holiday season.
Another example is the luxury Coffee Company, Nespresso. They partnered with our teams to uncover key consumer trends around the holiday season, including search trends, and consumer intent around holiday gifting, coffee recipes and seasonal flavors. With a better understanding of both auction dynamics and pinner behavior, Nespresso delivered effective advertising campaigns that also showed positive results and a third party brand lift study.
Turning to our preliminary outlook for Q1, we expect to grow revenue in the low 70s percent range in Q1.
As we think about Q1 in the full year, we expect positive trends from our investments in add tools like shopping and automation, and sales coverage expansion to continue. We plan to expand our international coverage further in existing geographies, and also expand monetization into Latin America in the first half of the year.
We also want to be mindful that we'll be navigating a fluid landscape, we're keeping an eye on a few things.
First, the impact of COVID on users and engagement. Since COVID appeared, we know that more and more people have come to Pinterest to find inspiration. We could see some of that reverse if restriction leaves.
Second, changes to privacy in the measurement landscape. This could impact the industry's ability to attribute conversions or serve relevant content.
Finally, before opening it up for questions, I want to touch on expenses.
We continue to navigate a more remote working environment while maintaining investments in the long term strategic priorities of the company. In the fourth quarter, we grew headcount 15%, year-over-year.
As we look to Q1, we expect to maintain our total non-GAAP operating expenses at its similar level compared to the fourth quarter driven by investments in our key strategic priorities, including content, engagement, advertiser success, and shopping. Thank you to the teams at Pinterest, our advertising partners, and all the people that come to Pinterest to find inspiration. And with that, we can open it up for questions.