Early pregnancy and very slow hours - when to disclose?

I am a midlevel associate and about 10 weeks pregnant. I had a great review last year and hit my hours goal, so I felt comfortable taking it easy while I could in January.

However, I soon found out I was pregnant and the symptoms have been hitting me HARD (extreme nausea, migraines, I can't function without a several-hour nap in the afternoon, cramps, heartburn, etc.). On top of symptoms, this pregnancy has not been easy - I had two episodes of severe bleeding and concerning early lab work that required way more appointments than is typical. At one point, I was scheduled for an abortion because I was told the pregnancy was nonviable, only to find a heartbeat once I rolled into the OR - I had scheduled 2 days off from work to have the abortion, but when it was canceled last minute, I still took the two days to recover emotionally. My pregnancy has tentatively turned around and my last several appointments have been normal/great, but I don't feel "safe" enough to tell anyone about it yet.

I told the head of my practice group that I had "an ongoing medical issue" that requires appointments throughout the week, and she totally fine with that. I haven't missed deadlines or turned in poor work, and no one has talked to me about my performance at all. However, I only billed 120 hours in January and will be lucky to bill the same in February. I haven't asked for work because I feel so sick, and I wanted to fly under the radar as long as possible until first trimester symptoms go away. But now I'm scared that I've been too slow for too long and that a talking-to is coming. I also have not started to feel any better yet.

I would appreciate any advice on whether I should just tell HR and my practice group lead ASAP about the pregnancy before someone comes to me first, and how I should frame this whole thing. Should I request a phone call? Send an email? How much detail should I include? What should the tone be? I don't want to be seen as blaming the pregnancy - many people in my group have been pregnant and continued to bill a ton and do great work, and I feel very inadequate/self-conscious about not being able to manage the same. I'm stressed and upset all around - obviously because of the difficult pregnancy, but also because I worked really hard to build a strong reputation at my firm and in my group and I feel like it's all about to crumble. Thanks in advance for any advice and for indulging my self-pity and anxiety.