The birth rate for twins in the United States in 2019 was 32.1 per 1,000 live births. The greater use of in vitro fertilization and other assisted reproductive technologies in recent years is thought to be connected with an increase in the twin birth rate. This figure includes twins born after using IVF or other ART methods. It does not include twins who were conceived naturally and then carried by their mother into pregnancy. That group numbers about 4% of all babies born. The rate of twin birth has been increasing since 1990, when it was at a record low of 29.5 twins per 1,000 live births. Since then it has risen almost 10%, to our current level of about 32 twins per 1,000 lives births. This rise may be due to more women having multiple embryos transferred during in vitro fertilization procedures. If this many embryos are transferred, they tend to be on average larger than those transferred in previous years, which could mean that they contain more than one fetus. Thus, the number of twins born after multiple embryo transfers would be expected to increase. There also has been some evidence that the rate of natural twinning has increased since 1990. This could be because couples are choosing to delay marriage until later in life, which is known to reduce the chance of having a singleton baby. Also, women are waiting longer before having children.