My first two children were always ahead of the milestone charts on every line item. I don't think they were "special" babies, I think the charts are somewhat of an average. So my personal opinion is that if a child is not hitting " a number" of the milestones by the stated time, they are delayed.
My 3rd child has issues, specifically showing up around the 5 month old mark. She got a booster and also had a cold- that cold went on for months and ear infections for 2 or 3 months. She would try to hold her bottle but she would end up knocking it out of her mouth. It was like her hand eye coordination was off. She sat up about a month late. Attempted crawling was months late, babbling around the 7 month mark just didn't seem to happen.(she was sick until 9 months) Both my other children cruised furniture and used a walker toy. This child didn't do either. She eventually got herself to a standing position and walked from that.She walked on time. She did say her first word at 8 months. As a toddler, words were spoken but not as many as my other two. Her vocabulary was about 25 words at two, my first child was 200 words by two. Speaking 2 or 3 words in a row was much delayed. Also multi syllable words were not attempted. My son said words like "unbelievable" by three yrs old, she wouldn't attempt this syllablation until 4 or even 5. Even a three syllable word was not attempted. My concerns fell on deaf ears with everyone, her pedi and my husband/friends. I was told she's fine and I was worrying for nothing. For riding a trike, she made it just "on" the cutoff date-36 months. People told me she would grow out of any delays but time does not make a child grow out of delays- at least from what I've seen. I beleive her delay to now be 2 or 3 yrs behind where she should be. She is 6 1/2 yrs old. Emotionally speaking, sometimes she doesn't respond to social cues exactly the way most people would. It's hard to put my finger on it exactly but there is just enough of it to know it is there. I taught all my kids to read, she took 3 times a long to memorize the letter sounds.
Trying to get her to sit on my lap to read a book to her was impossible until she was about 4. She just couldn't sit there or focus. A back and forth conversation was labored and many times in vain. I would say something to her and rather than respond to what was said she would return dialoge that was totally off topic.
Last year I was doing some medical research and had an AHA moment. I came across autism, and was shocked to see that she DID indeed, as a baby/toddler, Have all these same issues listed for this syndrome. Why a DOCTOR couldn't see it or diagnose it is beyond me.I am convinced she has autism.
Others might give me a smile and say things are fine, but being her mother, I know. A mothers instinct is a great thing! trust it.
Now, I have read several reports online that doctors have cured autism. Some have said it is caused by bacteria and used antibiotics/hyperbarics/bee venom, etc. Others have claimed internal fungal issues to be the cause. Still others claim it's a body laden in heavy metals or imbalances. If any of these have merit, I don't see how 'language therapy' would help and I think therapy seems to be the ongoing trend for many delays.
I think diet is certainly a BIG one for any delays, especially important to cut out processed food, sugar and carbs which supress the immune system and will help bacteria and fungus to grow.
My suggestion is to get yourself familiar with the different delay syndromes so that you can pinpoint any issues that might come up and get early intervention. If you have strong concerns and are blown off by a doctor, get a second opinion. Follow your instinct.