In accordance with the Employment contract of the Russian Federation, the employee can require the employer's renewal of fixed-term contracts. Let's say you are working on a temporary contract, learned of her pregnancy. Write to the head of the company's statement. It can be as follows: "the request to extend the term of the employment contract from (date) №__ to the end of pregnancy. The basis of medical certificate from (date) №__". Attach to the application a document confirming your pregnancy. A medical certificate must be issued correctly, have all required stamps and signatures.
You can also conclude with the employer the additional agreement. You should specify in it new term of the employment contract (it should not exceed five years). In any case, do not write the wording, which involves the extension of the term. Better mark this as a change the end date of a legal document. If you transfer to another position, for example, in the case of the regular worker, this should also be spelled out in the additional agreement. Be sure to specify the date of entry into force.
Note that if the head of the organization wants to modify contract terms, including the term, he must notify you no later than three days before the new conditions enter into force. If he didn't tell you about it, you can continue to work on a permanent basis.
According to the Labor code, to extend the term of the contract is impossible, so the best way in this case to use the wording "change of life". If the employer wants to continue working with the employee on a permanent basis, it is advisable to cancel an employment contract and sign a new indefinitely. But in this case, you must be compensated for unused vacation days.