Hi! I found this from Dr Google: (in information about IVF but it likely applies to pregnancy in general) "A thick, receptive, nourishing uterine lining is the best possible environment for the embryo, and the ideal lining is at least 7 to 8mm thick and displays a “trilaminar” (or “three layered”) appearance on an ultrasound."
My lining was 16.1 mm at 15 dpo, so I'm kind of glad I didn't google that until now (I'm 19 weeks) because I might have worried it was too thick! Ahh pregnancy, such a worrying time! I must add that 15 dpo was actually 6 whole days after the BFP for me).
I had a pregnancy of unknown location before this one, but I didn't ask how thick my lining was with that one, so unfortunately cannot compare it with this one. That PUL 'self-resolved' before anything ever showed up on a scan.
I had fairly ouchy one sided pains with both the PUL and this 19 week one that is certainly in the right place.....I guess ovaries can be grumpy buggers at times. This time around I didn't even ovulate from the same side as the pain...bodies are weird!
I know it's hard not to worry, but at 4 weeks, pregnancy of unknown location really just means they haven't spotted it on the scan yet. There's nothing visible at 4 weeks, at least not on the equipment the NHS early pregnancy units use (in my experience of one hospital, anyway).
Fingers crossed you have a beanie in the right spot and what they told you was just being over-cautious- they aren't allowed to give us hope unless they see certain things. Example, with this pregnancy, I had some bleeding and pain at the start, and the first Dr I saw told me there was very little chance of a good outcome before they'd even done the first blood draw for HCG...well she was wrong and the proof is kicking me as I type this.