At 18 weeks she needs more sleep than that. Assuming you don't have a wonder baby who is sleeping 14 hours a night without waking, she needs quite a lot more.
More sleep promotes better sleep. I am afraid you are caught in the opposite - that poor sleep spirals and means sleep quality just worsens and worsens.
The key things babies like to help them get to sleep is sucking and movement. You can give these several ways:
Sucking:
- Bottle feed to sleep
- Breastfeed to sleep
- Comfort breast suckle to sleep
- Dummy
Movement:
- Bouncy chair
- Sling/carrier
- car seat
- Pushchair
I favour dummy and bouncy chair, foot bouncing from the comfort of my sofa, or while sitting next to chair holding dummy in mouth until asleep.
At 18 weeks 20/30 minute naps are not unusual (although 30/40 minute nap would be better), but they should be frequent. Don't look at time asleep, consider time awake.
I would not expect baby to have longer than 90 minutes awake at this age. If it takes a long time to get baby to sleep then this might be reduced to 60 minutes to give half an hour to work at settling to sleep. I have always liked EASY (Eat, Activity, Sleep, You) on a 2-2.5 hourly structure:
Eat - Full Feed
Activity - 90 minutes from waking at most, or until tired signs are shown
Sleep - Aim for at least 30 minutes, or until wakes naturally
You time - then start again and repeat, repeat, repeat through to bedtime.