Mr Vazquez said he disagreed with the bill both philosophically and biologically
Uruguayan President Tabare Vazquez has vetoed a bill approved by parliament earlier this week to legalise abortion in the first 12 weeks of pregnancy.
He said it was more important to help women with unwanted pregnancies than to enable them to have abortions.
Tourism Minister Hector Lescano said the bill's backers lacked the majority they needed to overturn the veto.
Proponents argued that it would have reduced the number of women who die after resorting to illegal abortions.
Under the current law, women who have an abortion and the people who assist them face prison. Abortion is only allowed in the case of rape or when the life of the woman is in danger.
In a statement released on Friday explaining the veto, Mr Vazquez said: "Our laws cannot ignore the reality of the existence of human life in the gestation period, as scientific evidence clearly shows.
"It's more appropriate to look for a solution based on solidarity, giving a woman the freedom to make other choices and thereby save both her and the baby."
In a rare step in a Latin American country, the Uruguayan Senate voted on Tuesday to decriminalise abortion by 17 votes to 13.
The bill would have allowed abortion if there was a health risk to the mother or foetus. It would also have allowed a woman to end her pregnancy in the first 12 weeks under other circumstances, such as extreme poverty.
Moves to decriminalise abortion have provoked fierce debate in Uruguay
But two days later, as many of the bill's supporters expected, Mr Vazquez vetoed the legislation.
The president, himself a doctor, had earlier warned that he opposed the Law of Sexual and Reproductive Health because it included elements "with which I disagree, philosophically and biologically".
Opinion polls had suggested a majority of Uruguayans favoured easing their country's restrictions on abortion.
Most Latin American countries allow abortion only in cases of rape, when the woman's life is in danger or if the foetus is severely deformed.
But both Cuba and Mexico City, though not the rest of Mexico, allow abortions without restriction in the first 12 weeks.