In vitro fertilization (IVF) lab advances have improved how embryos are grown and selected. Your fertility team can now culture embryos to the blastocyst stage, which makes it easier to choose those with the best chance of success. Some embryos reach this stage on day 5, while others take more time and reach it on day 7.

Day 7 embryos are less likely to result in pregnancy, but they still have a meaningful chance. Here’s what you should know about why slower growing embryos may have lower success rates and why they still matter.

Timing and implantation

In the past, slower growing embryos were less likely to succeed in fresh transfers. This often happened because the timing between embryo growth and the uterine lining wasn’t aligned.

After ovulation, or after your egg retrieval, progesterone levels rise to prepare the uterine lining (the endometrium). The “window of implantation” is short — only a few days.

If an embryo doesn’t reach the blastocyst stage in time, the lining may no longer be ready. Today, most labs use extended culture and advanced freezing and thawing methods. With these, day 7 embryos can be frozen once they reach the blastocyst stage and then transferred later in a cycle where your uterine lining has been prepared at the right time.

Chromosome differences (aneuploidy)

Day 7 embryos are more likely to have aneuploidy, which means they have the wrong number of chromosomes. A healthy cell has 46 chromosomes. With aneuploidy, an embryo may have extra or missing ones.

Embryos with aneuploidy are less likely to implant or develop into a healthy pregnancy. In most cases, they do not result in a live birth.

Your reproductive endocrinologist may recommend preimplantation genetic testing for aneuploidy. In this test, a few cells are removed from the embryo before freezing, and the chromosomes are checked. By selecting embryos with the typical number, your care team can improve your chances of pregnancy and a healthy birth — even with day 7 embryos.

Other factors that may play a role

Even when your uterine lining is prepared at the right time and your embryo has the correct number of chromosomes, day 7 embryos still tend to have lower pregnancy rates. Embryos that reach the blastocyst stage earlier, on day 5 or day 6, are more likely to result in pregnancy.

Researchers are still learning why this happens. Some studies suggest that day 7 embryos may function differently at the genetic level compared to faster growing embryos. Scientists continue to explore:

• What factors cause some embryos to grow more slowly
• Whether lab techniques or stimulation can improve these outcomes
• What slow-growing embryos can teach us about fertility overall

Why day 7 embryos still matter

Even with lower pregnancy rates, day 7 embryos can result in healthy pregnancies. These embryos are especially important if you have only a few available. Day 7 embryos can add to the number that can be frozen and later used in treatment.

With extended culture, advanced freezing, genetic testing, and carefully timed transfers, these embryos have the best possible chance of success. For many patients, that means another opportunity to grow their family.

If you have questions or concerns, Progyny is here for you. Please contact your Progyny Care Advocate for support.

Disclaimer: The information provided by Progyny is for educational purposes only and is not medical advice. Always consult a qualified healthcare provider for medical guidance.

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When people talk about age and fertility, most of the focus is on eggs. As you get older, egg quality declines, which makes it harder to conceive and raises the risk of miscarriage. But researchers are also studying how the uterus itself may change over time — a process sometimes called uterine aging.

What is uterine aging?

Uterine aging refers to natural changes in the muscles, blood vessels, and lining of the uterus that occur with age. These changes may affect:

• How the uterus contracts during labor
• How well it supports a pregnancy
• How the lining responds to an embryo during implantation

Some studies suggest that with age, the lining of the uterus (endometrium) can become thinner and less receptive, and inflammation inside the uterus may increase. These changes do not make pregnancy impossible, but they may lower the chances of implantation or increase the risk of complications.

Egg quality vs. uterine health

Egg quality remains the main factor in age-related fertility decline. Chromosomal changes in eggs are well documented and account for most of the reduced success rates seen with advanced age.

Even so, research has found that pregnancy rates are sometimes lower in older people using healthy donor eggs, suggesting the uterus may also play a role. Some studies show that the uterine lining becomes less effective at supporting implantation over time, while others find no difference once embryo quality is controlled for.

This ongoing debate highlights how complex fertility can be. While eggs are the biggest factor, uterine health may matter too.

Other age-related health factors

Age also brings more opportunities for other health conditions to develop. These may affect fertility and pregnancy outcomes, including:

• Fibroids, adenomyosis, or endometriosis
• Uterine scar tissue from prior surgeries
• Diabetes and insulin resistance
• High blood pressure or cardiovascular disease
• Autoimmune conditions

These issues can contribute to lower live birth rates or higher risks during pregnancy.

Risks of pregnancy at advanced age

Pregnancies later in life, especially after age 45, are more likely to involve:

• Gestational diabetes
• High blood pressure or preeclampsia
• Placental complications
• Preterm birth
• Cesarean delivery

Babies born to older parents may also need more specialized care after delivery, such as time in a neonatal intensive care unit.

Can uterine aging be treated?

Scientists are exploring potential ways to slow or reverse uterine aging, such as medications that reduce inflammation or oxidative stress, and stem cell therapies. These treatments are still experimental and not part of routine fertility care.

For now, the best step you can take is to optimize your overall health before trying to conceive. Talk with your doctor about your medical history, fertility goals, and whether a uterine evaluation may help guide your care.

Key takeaway

Egg quality is the biggest factor in fertility as you age, but the uterus may also change in ways that affect pregnancy. By working closely with your care team and addressing both egg and uterine health, you can make informed decisions about family building and improve your chances of a healthy pregnancy.

If you have questions or concerns, Progyny is here for you. Please contact your Progyny Care Advocate for support.

Disclaimer: The information provided by Progyny is for educational purposes only and is not medical advice. Always consult a qualified healthcare provider for medical guidance.

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