My daughter was 3 when I was pregnant with my 2nd child.
My kids are spaced 4 years apart.
Just so happened.
It is perfect.
I, when pregnant, did not make it 'my' pregnancy, but incorporated her into it and explained, Mommy's pregnancy 'phases'. I explained I get tired, tummy grows, I will need to nap etc., that we will take photos of her each month with my growing belly which she loved, that she is always 'my first baby...', she sang to her baby brother in my tummy, talked to him, we napped together etc. So, all in all, she was bonded... with her baby brother already and used to him, even before he came home from the hospital. It is, 'prepping' your eldest child, so that they adjust, well. Once baby comes home.
Explaining that once home, a baby will cry, wake, I will nurse him, but it is Mommy's 'job' and she does not have to worry nor do I expect her to act older or more perfect, than she is.
So, it is prep work, for your eldest, while you are pregnant, and after.
So that they adjust to "their" new baby too, in the house and them not being an only child, for those previous years.
Having 2 kids... will be busier.
My Advice is:
The Husband has to help, more. If he is not already with the one child.
You will need to be organized.
Have a routine, daily for both.
TALK to your Husband, before baby is born and while you are pregnant, so that he knows, that he has to help more and spend time with the eldest... too. And so that, the eldest will be used to that. And per bedtimes etc., one Spouse will need to get the older one to bed, and you with the infant etc.
My daughter LOVES having a sibling. Loves her little brother. When he was born she told everyone "Mommy had brother for me...." even if that is not what we told her. But that is how she felt. It was real heartwarming.
She adores, her sibling.
AND, the upshot was, that she was old enough by then... to understand things better, and it was easier for me... that way. Because, she was not still in diapers and what not.
Having 2 kids is great.
Our family is complete.
For the kids and us.