As children get older their rate of vocabulary growth increases. Children probably understand their first 50 words before they produce them. By the age of eighteen months, children typically attain a vocabulary of 50 words in production, and between two and three times greater in comprehension. A switch from an early stage of slow vocabulary growth to a later stage of faster growth is referred to as the vocabulary spurt. Young toddlers acquire one to three words per month. A vocabulary spurt often occurs over time as the number of words learned accelerates. It is believed that most children add about 10 to 20 new words a week. Between the ages of 18 to 24 months, children learn how to combine two words such as no bye-bye and more please. Three-word and four-word combinations appear when most of the child’s utterances are two-word productions. In addition, children are able to form conjoined sentences, using and. This suggests that there is a vocabulary spurt between the time that the child’s first word appears, and when the child is able to form more than two words, and eventually, sentences. However, there have been arguments as to whether or not there is a spurt in acquisition of words. In one study of 38 children, only five of the children had an inflection point in their rate of word acquisition as opposed to a quadratic growth. This study suggests that most children do not have a vocabulary spurt.