For me (and I've heard the same for other moms with large age gaps) having a 2nd baby was more like having another first baby because they were 7.5 years apart. He might feel the same. 

I’m having a baby with DH & he’s incredibly excited about it. Having done it before doesn’t necessarily mean he won’t be excited to do it with YOU. 

I remember having those fears years before we conceived & telling my mum I was worried he wasn’t going to be as excited or think it’s special when we have ours. My mum has 4 kids, and she just said ‘monkeysee that’s ridiculous, every baby is special. The first isn’t more loved than the last, so quit thinking that way’. My little sister, her 4th, was an oops baby, and my mum said she actually enjoyed that pregnancy & baby stage the most because she was more confident & had more help than she’d had with the previous 3.

Don’t let his past take away your excitement at having your own child. 

to be honest I’m more concerned with you calling your skid “unwanted” more than I worry that your husband won’t hire a marching band to celebrate his second child. 

You’re not worried about your DH being “stripped” of his feelings, you’re worried that your skid’s unwanted existence will take the attention away from your baby. 

I have a very similar situation. My SD is 11 and was also unplanned - DH and BM were never married, I believe BM got pregnant to try to keep DH around, DH claims he wanted an abortion but BM wouldn't do it (I know there are 2 sides to every story but this is what I was told - I don't really care either way). Anyway, she's here now and we raised her most of her life because BM's little plan didn't work and so she didn't have any need for SD anymore.

ANYWAY, we have a 3 year old BD together and are expecting our second. I obviously wasn't there when BM was pregnant or when SD was born but I can say even with our 2 PLANNED pregnancies, they are still both different. This leads me to believe every pregnancy and birth is different, no matter what the circumstances are. It took 2 years of fertility treatments to get pregnant with our first BD. Our second happened right away.

I will say, DH always makes comments about how nice and different it is for BD (compared to SD) to have a real mother.

Try not to worry about this. Once your baby is here and you see everything fall into place, you won't even give this another thought.

If you have consensual sex with someone, any resultant baby is generally NOT unplanned.
It is your responsibility and consequences - especially as a man - to know the results of believing a woman who says she is on the pill, even if she is your wife.
If he didn't use a condom, it is not a case for a child being unplanned.
Yes, birth control can fail, but most stories about unplanned babies beggars belief IMO.

That said, yes,  your pregnancy will be different and special to both of you.
Let go of comparisons to his first wife and first child, it is hurtful to you.
You can't undo what happened in the past,but you can go forward and make your upcoming marriage and pregnancy special and meaningful to both of you.
Let go of the past: focus on your future with the man you love and who loves you back.
What you choose to make of any moments in your life will be by your own design.

In my case it was no less special. DH wasn’t in love with his ex but married her of pressure and tried to stick it out. They had 2 girls that he adores. 

I have BD16, BD14, BD11, SD 16, and SD11. When DH and I had our baby...she is now 1. It was just as exciting and new for both of us because she was OURS. 

Trust me of all the things to worry about in blended families I would put this one on the back burner.

she kidnapped the 23 & 14 yr old 6 yrs ago (sd’s) and hubby had sole custody of ss20 since divorce

for hubby because of the pas bio mum inflicted, even ss never truly had a real father-child relationship with hubby. 

The gap between the youngest of his and mine will be like 11 years and  hubby told his family he actually got to enjoy and experience having a wife pregnant.

when ex got pregnant, she did everything to sabotage the pregnancy or getting pregnant. She went to appt alone, hubby was banned from coming, hubby was not involved then the day bubs was due or she admittedly was in early stages of labor turned into a psychotic screaming match from ex wife telling off drs to “cut the baby out of her” and hubby would tell her off csection is serious abdominal surgery, not an oopsies moment where you just demand it because you don’t wanna deal with labor and pushing. When the 3rd child was about to be born she told the dr to get her tubes tied, hubby said to dr just do what the hell she wants. Hubby knew then he was headed for divorce that this isn’t how normal marriages/relationships should be

when we got married, every pregnancy appt hubby came, he was involved with those major decisions but he already knew my standpoint on the birthplan, vaginal birth, minimal pain med and csection only if emergency and he supported it.

when our daughter was born hubby said it was a new experience seeing me push and actually dealing with labor because his family knew exwife had 3 csections, hubby kept saying i was crying in so much pain and baby comes out and i’m just normal like i didn’t just give birth a short while ago and hubbys sis said some women are just like that

bare in mind exwife did everything to brainwash, torture and alienate the 3 skids so there is no proper relationship between them and dad but they do not feel this because they think its normal.

its only our 2 kids that hubby has a proper interactive relationship but skids try to guilt him about it but they do nothing to maintain a relationship 

often people dealing with a divorce, are relieved to meet an amazing person and go through subsequent pregnancies with them so don’t automatically think your pregnancies mean nothing. We are 2 kids now and will possibly have another 1 or 2. Anytime hubby thinks i’m pregnant he is excited 

considering how batshit crazy exwife is, the narcissm and her lack of boundaries with us, i don’t give her the satisfaction of worrying that my kids are not important compared to hers 

The whole lamenting the "firsts" thing is a chronic topic for many Sparents. Particularly SMoms.  It does not compute for me ... at all.

I am a pragmatist.  I am also a man.  My perspective on this topic is that having a child with you will be his first. His first with you.  Every day and everything that you experience together will be the next first..... together.

Another pragmatic perspective that I have relied on in my marriage is that our past and experiences are what make us who we are.   If my bride had not had our son (my SS now adopted) as a 16yo teen mom she would not be the woman that she is.  The woman that amazes me every day, who I love beyond measure, and who I can't wait to spend the next day together with. 

Her first child is our first child.  I may have been his SDad from before he was 2yo until he was 22 but I raised him as my own and when he was 22 asked me to adopt him. So we made that happen.

His mom and I will celebrate our 25th anniversary in a few months. Every minute has been a new minute together.

So, please do not sabotage your happiness and your relationship over the past. Neither his nor yours.   

As for BM, all she has of your STBDH is his past. You are his today and his future.  If you keep your head on straight your children will be part of that as well.

Enjoy it.