useEffect is running on initial load & on update – How do I limit to just changes?

In my code example, my useEffect is subscribed to postId, which is not changing at any point. Yet useEffect is still being run.

Is there a way to prevent this from running and only launch if I change it?

import React, {useState, useEffect} from 'react';
import Vim from './Vim';
import './Main.css';


function Main():JSX.Element { 

  const [postId,updatePostId] = useState<number|null>(null)
  const [content, updateContent] = useState<string>('default text');
  const apiUrl = 'http://127.0.0.1:8000/'

  useEffect(()=>{
    // Detect change in PostID & go download it.
    // Ignore if we are coming from null->number
    console.log('Why am I running?')
    fetch(apiUrl+'get_post/'+postId)
    .then(res=>res.json())
    .then(result=>console.log(result))
  },[postId])


  function loadPost(pid:number):string|null{
    // fetch from API, load post content
    console.log('I can access:'+postId)
    return null;
  }
  
  function backLinks():JSX.Element{
    
    return(
      <div className="backlinks">
      </div>
    )
  }

  return (
    <div className='main'>
      <Vim content={content} />
    </div>
  )
}

export default Main