1. Introduction
Skills acquisition in sentence producing is among the criteria for speech development [2]. By developing and increasing the age, we are witnessing the emergence of compound sentences in speech [4]. Clinical treatments for language disorders in Iran are primarily based on the experiences of speech-language therapists or per English language norms. We need to examine how the length of utterance and compound sentences develop in Persian. The present study aimed to report the development of compound sentences and Mean Length of Utterance (MLU) for Persian-speaking children aged 15-34 months.
2. Materials & Methods
 In this longitudinal case report, based on the Developmental-Descriptive model, MLU and emergence of simple compound sentences of a Persian speaking girl sampled and described from 15 to 34 months by daily scripting. If a complete sentence produced by the child had a verb, it was considered a simple sentence, and if there was more than one verb, it was considered a compound sentence [21].
The MLU calculation rules in the child’s spontaneous speech sample were first, samples of the child’s recorded daily speech over time were divided into utterances. The utterances were then scripted based on the international phonetic alphabet, and each utterance was scripted in one line. The number of words per utterance was calculated; whether content or grammar, each word was counted as one word. Finally, all the words counted in the speech sample were added together and divided by the number of utterances [12, 13]. R software was used for data analysis. The correlation of age with linguistic variables was evaluated by the Pearson correlation coefficient. 
3. Results
At the age of 21 months and 13 days, the examined child uttered her first simple two-word sentence, and at the period of 26 months and 29 days, she used a four-word compound sentence. With the emergence of compound sentences, the increase in the MLU has gained considerable speed. In a few days, we observed the addition of a unit to the length of the sentence (word). There was a significant relationship between the MLU and age (r=0.925, P=0.001), the mean length of sentence, and number of words in the Persian-speaking child (r=0.910, P=0.002), and the total number of words and age (r=0.928, P<0.001). 
According to Tables 1 and 2, From the age of 27 months, with the advent of compound sentences, the mean Length of the Utterance word has increased significantly.




In a few days, we observed the addition of a unit to the utterance (word).
4. Discussion & Conclusion 
The present study presented a report on the acquisition and development of the child’s MLU of utterance based on the descriptive-evolutionary model [12]. An attempt was made to examine the questions raised through daily scripting to achieve this goal.
Although several longitudinal studies have been conducted on the development of MLU and the number of words in other languages, similar studies are rare in Persian. It is crucial to conduct investigations of this type to make cross-linguistic comparisons and study the linguistic universality and specificity. By increasing age, we see increasing the number of words in the child’s vocabulary, the emergence of compound sentences, and increased MLU. At 27 to 28 months, there is a dramatic change in expressive language development as the compound sentences appear. The emergence of compound sentences in the language output also plays a significant role in increasing the MLU and can be considered in clinical language assessments.

Ethical Considerations
Compliance with ethical guidelines
In this paper, the principles of publication ethics were considered.

Funding
This article has no financial support. 

Authors' contributions
Conceptualization: Manijeh Zarei Mahmoudabadi; Data analysis: Manijeh Zarei Mahmoudabadi, Talieh Zarifian; Text writing and review: Manijeh Zarei Mahmoudabadi, Talieh Zarifian, Rabab Teymouri. 

Conflicts of interest
The authors declared no conflicts of interest.

Acknowledgements
The authors express their appreciation to Mr. Amir Hossein Rashidi.



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