Libby Kranz with her daughters Jennifer (left) and Charlotte

Unable to have genetic children due to Liz's diminished ovarian reserve, the couple adopted frozen embryos – the products of two other couples' IVF cycles. After two attempts at pregnancy and two miscarriages with the adopted embryos, the third attempt was a success, and Krainman welcomed a healthy baby girl, Sammy, in July. Krainman, 33, remains in touch with Libby Kranz, 35, the woman who donated the embryo that resulted in her daughter.

Liz shared her incredible journey to motherhood here. Below, her embryo donor opens up to PEOPLE exclusively about her own story, including the tragic loss of her 6-year-old daughter Jennifer, her bond with Krainman, and the relationship she and her family might one day have with Sammy.

Kranz and her husband Tony adopted their daughter Jennifer at birth in 2007 after five attempts at IVF resulted in five miscarriages. They were planning to adopt again when Kranz became pregnant naturally with the first of three more children: The family welcomed Jonathan, now 5, Nicholas, now 3, and Charlotte, now 1. But they still had four frozen embryos from IVF that they "weren't comfortable destroying," says Libby. "I believe they were life. I believe those were our children at some stage."

Libby Kranz (in blue) with her husband Tony and children, from left: Jonathan, Nicholas, Charlotte and Jennifer

Zemya Photography

Having completed their family with four children, the couple decided to place their remaining embryos up for adoption. They found Krainman, and she got pregnant.

Shortly thereafter, Kranz's 6-year-old Jennifer was diagnosed with DIPG, a terminal brain tumor. She would not live to see Sammy's birth. Nine months after her daughter's death, Kranz is a staunch advocate for pediatric cancer research funding and the co-founder of Unravelpediatriccancer.org.

Here, in her own words, Kranz talks about her part in the journey to helping Krainman become a mother.

How did you find out donating your embryos to another couple was even an option?
You only really hear about destroying them or donating them to science. I knew those weren't an option for us, and I didn't really like the idea of donating them in a random way. So I said, well, wait a minute. There's got to be something more. And what we ended up doing was much more of a quasi-traditional adoption.

How did you meet Liz?
I was on a traditional adoption forum and someone asked, "Have any of you looked into embryo adoption?" And I said, "Not for myself, but I do have four frozen embryos that we've always planned on placing." This woman asked if she could share my information on an embryo adoption forum and I said yes. We got 10 families that wrote back. And for whatever reason, Liz quickly rose to the top. Liz and I talked pretty seriously, and then I told my husband. At first, he was hesitant about it, even though we knew we were done with four kids. For him, it was like, "Wait a minute, this is huge!" As he read emails back and forth between me and Liz, and saw how I felt about her, he was completely on board.

Liz Krainman was able to get pregnant thanks to Libby Kranz's donated embryos

Courtesy Liz Krainman

Can you describe what it was about Liz that made you realize she was the one?
No, because it was just a feeling. She didn't have some of the criteria, frankly, that I cared about. They didn't have a big family, and she wasn't a stay-at-home mom [at the time], which, if you'd asked me, those would have been my top criteria. But the whole reason we wanted to do this [when we did] was for Jennifer. Jennifer was starting to question where she came from and had questions about adoption, and I thought, wouldn't this be phenomenal for her to see that families can be built in all sorts of ways? She was truly the catalyst for us.

And I liked Liz. I wished we lived closer, we could go out and get a drink, have coffee. It's not the superficial things that make good parents. It just felt right. And I thought Jennifer would like them.

How did you feel during Liz's pregnancy not knowing whether the baby would be from your embryo or the embryo from the other donors?
Truthfully, it was the least of my concerns because [Jennifer got sick] very quickly after Liz got pregnant. We knew we would lose Jennifer with her type of cancer. But Jennifer would have loved it. Liz and I believe that somehow Jennifer met Sammy before the rest of us did. I think it would have meant the world to Jennifer to see that families can be built in different ways and that her mommy and daddy did this.

Jennifer with her sister Charlotte

Courtesy Libby Kranz

What's it like for you now that Sammy is here and your embryo is now another family's child?
There are days that it's difficult, for sure. But it's because of what we're living through losing Jennifer. This should have been something I was posting on Facebook. And saying, "Hey people, this is an option. Do you know about this? It's worth doing. This is the good that's come from it for my family." This should be a bigger presence in my life. This was a big deal, and it was hard at every step. It wasn't easy. But some of the best things you do in your life are some of the hardest things you do.

I think the hardest part for me is that Sammy is a full genetic sibling to our children. I look at her and I'm like, who does she look like? And it's a very surreal thing. But I don't think she looks like any of us. I think maybe that's somehow from Liz. Liz and I are friends on Facebook, and we've talked very openly about how there may be a point in time where I have to block her because I can't do it. We both know there's the potential for that. I will say, every single time she posts a photo, I'll stop on a picture, and for a split second I won't realize whose picture I'm stopping on. I'll be like, cute baby … oh, God!

My honest thought is just that I am so glad such amazing people are parents. That is one loved little girl. This was, without a doubt, the right choice.

Liz Krainman (right) was able to get pregnant with an embryo donated by Kranz. Liz and her husband Kevin (left) welcomed baby Sammy (center) in July

J Shumway Photography

Down the road, what sort of relationship do you think you'll have with the Krainmans?
As things in my life calm down a little bit more, I hope that Liz and I can be friends. I think she's a cool chick. I think I could help her because we have similar parenting styles, just by nature. When she had the earlier miscarriage, I talked to her. I talked to her when she was pregnant. I loved it, so I hope we can get back to that point, but that's on me and not her. The kids will know each other somewhat. I always have this vision of us meeting somewhere. I'd love for the kids to meet one day. I'll be interested as Sammy gets older to see things like: Two of our children walked early. Will she? The nature versus nurture portion of everything. But my hope is that the kids will know each other as much as any of them want to.

How will you feel if Sammy wants to have more of a relationship with you?
I think I'll be fine with that, especially by then. Because now, my primary concern is my other kids. But I will always make sure our information gets to Liz. One hundred percent, Sammy has every right to know everything and anything she wants to know. She can come visit us. Her needs will always come first.
What do you want others to know about embryo adoption?
I think people don't realize this is an option. Even people who do IVF. And that was kind of my plan, that we would tell people about this. And what a gift it is, because I gave that child the gift of life, literally. And these parents the gift of parenthood, and Liz the gift of pregnancy. I always wanted the experience of being pregnant, so I know how much it meant to me. To give that to her was a huge portion of this.

What I've learned about the embryo adoption world is that we're more rare, and oftentimes this is done anonymously or it's done from donor sperm or donor egg. It's rare that couples have their own genetic material with other siblings and still place them. I get it – it's hard to do.

The biggest thing I want people to know is this is not just a gift for them, I do believe it's a gift to us. It's something I'm proud of. That's a good feeling, to say, Hey, I did something good. I helped someone, I did something that wasn't easy to do, but it felt like the right thing. And it's a way to put my money where my mouth is, especially with Jennifer. To say: Biology is not what makes a child your own.

To get involved with Kranz's pediatric cancer charity, click here. And for an exclusive interview with Krainman's doctor, check back on PEOPLE.com Wednesday