Sarah posted a slideshow of photos in which she and Dylan kissed and embraced at sunset while holding sonogram images of the baby.
"Our embryo transfer was a success and I am pregnant with our baby boy! Our little 5day embryo nestled in comfortably... As you may have suspected, we received our first positive HCG test within hours of me breaking my knee," Sarah wrote in the caption.
"Two days later, my betas doubled, and this week we saw our baby's heartbeat flicker for the first time. Surreal doesn't begin to describe how it's all felt. I've spent the first 3.5 weeks of my pregnancy on bed rest, nourishing myself and putting all of my energy into growing bones and organs."
Sarah said she's been on "a miraculous path" after those two events "coincided for a reason."
"The road ahead is long and I know we'll be holding our breath until the day our baby is earth side with us," Sarah continued in her post.
"I've dreamt of the day that I could share this news, and at the same time I've dreaded how I would word it. To the women and families amidst their fertility challenges; please know that I share this news with the most gentle intention."
Sarah said whatever feelings come up for those women are "valid and respected."
"My heart belongs to all of you today. We are all warriors and I assure you I am still in it with you. I hope this lessens the sting, though I understand it may not," Sarah insisted, before thanking her doctor.
"As for now, there's so much I've wanted to tell you and ask you about... where do we begin?!"
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Sarah has shared her fertility journey with fans since Day 1.
"One of the blessings of IVF is the support I have gained by being transparent about this process," Sarah toldUs Weekly in an interview.
"We have been trying to get pregnant for exactly two years. We started trying to conceive naturally in 2020, but after seven months of no luck, we turned to fertility testing which confirmed that I had Diminished Ovarian Reserve. As soon as we learned my diagnosis, we immediately turned to IVF."
Sarah told that magazine she and Dylan are "protecting" their hearts and expectations in case anything unforeseen happens.
"It's admittedly overwhelming to think about," Sarah shared. "We are excited and hopeful for our near future, but also realize that we are still in the first trimester and have a long road ahead of us."
Sarah also gushed about how she's "unbelievably grateful" to have had a supportive partner throughout the entire process.
"Dylan has been an incredible rock and is strong for both of us when it has felt too heavy at times. He's a rugged man, but so tender at heart," Sarah said.
Sarah apparently looks forward to receiving advice from Bachelor Nation's Ashley Iaconetti, and she said she's most looking forward to gaining perspective and appreciating her pregnant body, before and after she delivers.
"As someone who has struggled with body image, I know with certainty this area of change will be difficult for me, but I feel safe and prepared for what lies ahead!" Sarah told Us.
Sarah announced in May 2021 that she's engaged to Dylan after four years of dating.
"We are thrilled to share the announcement of our engagement! In our favorite place, with close friends and Rio, Dylan popped the question in front of Mount Sopris," Sarah captioned a slideshow of photos of the marriage proposal on the rocks by the water.
Sarah and Dylan were apparently joined by their dog Rio and close friends in the Colorado mountains.
"To everyone who thinks life has to go in one particular order, or by a specific time... IT DOESN'T. I'd wait a lifetime all over again -- through the heartbreaks, years of self-work and countless Rose Ceremonies -- to end up with this person."
Sarah continued in her post, "We make each other better, we have so much fun and we're going to become parents together, so we decided to do the damn thing!"
"5.15.21 My forever paradise, favorite adventure bud and BEST friend, @dylan.h.brown," she concluded.
For now, however, the couple's wedding plans are on hold.
"We feel that we have enough on our plates right now and will resume planning once baby is here," Sarah told the magazine. "The idea of our little baby getting to attend our wedding with mom and dad someday feels really beautiful to me, so we'll wait until that day is right."
Sarah, who competed on Sean Lowe's The Bachelor season in 2013, began dating the filmmaker in March 2017.
One month later, Sarah and Dylan went Instagram official when she shared a picture of the couple and gushed, "Happily not returning to Bachelor in Paradise this summer." She added the hashtags "Paradise Found" and "adventurebuds" to her post at the time.
Sarah chose to date Daniel, a wise-cracking goofball, instead of Christian -- a decision many fans criticized because Christian had treated Sarah wonderfully on a date in Mexico and seemed to have honorable dating intentions.
Although Daniel did appear to develop genuine feelings for Sarah, he admitted to cameras he just needed a rose in order to stay on the ABC reality dating show longer.
Once it became time for Daniel to hand a rose to Sarah, he gave it to Haley Ferguson instead. Haley one of the bubbly blonde twins from Ben Higgins' season of The Bachelor, is now married.