Gross margin decreased to 72% for the nine months ended September 30, 2021, compared to 75% for the nine months ended September 30, 2020. Gross margin for the nine months ended September 30, 2021 was lower due to a larger percentage of our revenue units coming from full systems, which require an IPG and a stimulation lead, as compared to individual IPG sales. This was partially offset by an increase in the average selling price.
Research and development expenses
R&D expenses decreased $0.2 million, or 3%, to $5.7 million for the nine months ended September 30, 2021, compared to the nine months ended September 30, 2020. This was primarily driven by a $0.8 million decline in clinical study expenses, partially offset by a $0.3 million increase in compensation expenses, including salaries and other employee-related expenses, mainly as a result of increased headcount, and a $0.3 million increase in non-cash stock-based compensation expense.
Selling, general and administrative expenses
SG&A expenses increased $11.7 million, or 182%, to $18.1 million for the nine months ended September 30, 2021, compared to the nine months ended September 30, 2020. This was driven by an increase of $6.3 million in compensation expenses, including salaries and commissions, and other employee-related expenses, mainly as a result of increased headcount, a $1.9 million increase in marketing and advertising expenses, primarily related to the commercialization of our BAROSTIM NEO in the U.S., $1.4 million of additional travel expenses, a $1.0 million increase in consulting expenses, and a $0.8 million increase in non-cash stock-based compensation expense.
Interest expense
Interest expense remained consistent at $1.8 million for each of the nine months ended September 30, 2021 and 2020.
Other (expense) income, net
Other expense, net was $13.4 million for the nine months ended September 30, 2021, compared to other income, net of $0.6 million for the nine months ended September 30, 2020. This change was primarily driven by a $13.7 million increase in expense related to the increase in fair value of our convertible preferred stock warrants due to the change in our common stock price from January 1, 2021 to July 2, 2021, which is the date the warrants converted to common stock warrants.
Provision for income taxes
Provision for income taxes was nominal for each of the nine months ended September 30, 2021 and 2020.
Liquidity, capital resources and plan of operations
We have incurred significant operating losses and negative cash flows from operations since our inception, and we anticipate that we will incur significant losses for at least the next several years. As of September 30, 2021 and December 31, 2020, we had cash and cash equivalents of $170.9 million and $59.1 million, respectively. For the three months ended September 30, 2021 and 2020, our net losses were $6.1 million and $3.2 million, respectively. For the nine months ended September 30, 2021 and 2020, our net losses were $32.5 million and $10.7 million, respectively. Our net cash used in operating activities for the nine months ended September 30, 2021 and 2020 was $20.7 million and $12.0 million, respectively.
Prior to the IPO, our operations were financed primarily by aggregate net proceeds from the sale of our convertible preferred stock of $383.1 million, as well as debt financings. In July 2020, we completed an equity financing pursuant to which we issued 62,500,000 shares of Series G Preferred Shares at a price of $0.80 per share, for net proceeds of $49.8 million after deducting offering expenses. In September 2019, we