Here's the list. Welcome to MN.
ADHD
attention deficit & hyperactivity disorder
AF
Auntie Flo (period)
AFAIK
as far as I know
AIBU
am I being unreasonable?
AS
Asperger's syndrome
ASD
autistic spectrum disorder
ATM
at the moment
BBS
be back soon
BC
before children
BD
baby dancing (having sex)
BF
breastfeeding (or best friend or boyfriend depending on context)
BFP
big fat positive (refers to a pregnancy test)
BIL
brother-in-law
BRB
be right back
BTW
by the way
bump
Not an acronym of course. People sometimes post a message saying just "bump" - as a way of getting a thread back into the active conversations list so that more people see it. Any posting will "bump" a thread back up the list.
CAT
contact another talker - use the Mumsnet contact another talker faciliity to contact someone - as in "Why don't you CAT me?"
CF
cystic fibrosis
CP
cerebral palsy
DD
darling/dear daughter
DGS/DGD
darling/dear grandson/granddaughter
DH
darling/dear husband
DLA
disability living allowance
DP
darling/dear partner
DS
darling/dear son
DS
Down's syndrome
DSS/DSD
darling/dear stepson/stepdaughter
DW
darling/dear wife
FFS
for f**k's sake
FIL
father-in-law
FWIW
for what it's worth
FYI
for your information
HTH
hope this helps
IEP
individual educational plan
IIRC
if I remember correctly
IKWYM
I know what you mean
IME
in my experience
IMHO
in my humble opinion
IMO
in my opinion
IYKWIM
if you know what I mean
IYSWIM
if you see what I mean
LMP
last menstrual period
LOL
laugh out loud
MIL
mother-in-law
MN
Mumsnet of course! MNer is a mumsnetter
NT
neurologically typical
OT
occupational therapy/therapist
PFB
precious first born
PG
pregnant
PIL
parents-in-law
PITA
pain in the a**e
PMSL
p**s myself laughing
RL
real life
ROFL
roll on floor laughing
SAHM
stay at home mother
SALT
speech and language therapy/therapist
SENCO
special educational needs co-ordinator
SIL
sister-in-law
SN
special needs
SPD
symphisis pubis dysfunction
TBH
to be honest
TIA
thanks in advance
TMI
too much information
TTC
trying to conceive
VBAC
vaginal birth after c-section
WB
welcome back
WTF?
what the f**k?
WOHM
working out of the home mother
YABU
You are being unreasonable
YANBU
You are not being unreasonable