Waterbirths, baby aspirin, and hair dye

Welcome to the weekly ZIP - your Zenith Informed Pregnancy!

Read on for a quick zip through 3 of the week’s most popular pregnancy questions, and the evidence behind them. Plus - bonus content on the latest & greatest in the world of pregnancy research. 

This week's top pregnancy questions:

1️⃣ Thinking about a waterbirth, what to know?

2️⃣ Is it true that I should avoid dyeing my hair during pregnancy?

3️⃣ What’s baby aspirin, how is it different from normal aspirin, and why would my doctor suggest I take it?

Thinking about a waterbirth, what to know?

With the desire for a “natural” or intervention-free birth a goal for lots of moms, laboring and birth in water often comes up as an option. It’s been around forever (first reported in the early 1800s!) and comes with a lot of misconceptions - is it only for homebirths? Is it actually safe to deliver a baby in water? There’s a lot to be clarified by looking at the data, and understanding the waterbirth protocols of your specific care team and facility. 

📚The tl;dr from the evidence:  The term “waterbirth” can generally refer to two things that are actually distinct - laboring in water (for example, getting into a tub during contractions) and delivering in water (where your baby is actually born in water). The data is strongest on laboring in water, but both laboring and delivering in water are generally considered to be safe and potentially beneficial options for low-risk, full term pregnancies in an appropriately prepared setting. And, many hospitals now include tubs or showers for laboring, so interest in waterbirth is not limited to homebirths!

For many women, the data shows that being in warm water while laboring is a strong comfort measure with lots of benefits - with less reported pain, less need for pain medication, higher reported birth satisfaction, and even sometimes a shorter labor and less tearing. Studies on waterbirth have generally not shown an increased risk of adverse outcomes for mom or baby, other than an increased risk of cord avulsion (the snapping of the umbilical cord) when delivering in water - resulting from the baby being lifted out of the water too quickly (and/or a short umbilical cord). With experienced providers, this risk can be minimized by carefully and slowly bringing the baby to the surface, avoiding excessive pulling/tugging on the cord, and immediately clamping the cord/monitoring the baby if it does snap. 

The primary practical considerations that exist with water are infection risk (which is highly dependent on setup and cleaning practices of the specific tub/location you’ll be laboring in), potential overheating or dehydration, accurately estimating blood loss, or slips/falls getting in or out of the tub. These can all be managed effectively with strong procedures for cleaning and appropriate temperature management of the water, and clear protocols for moving out of the tub in case something changes or in case of an unexpected emergency. 

As you’re considering it, you can ask your OB or midwife (whoever will be supporting your delivery) some questions to help you decide what’s right for you – like:

• Do you support laboring in water only, or also delivering?
• How will my baby be monitored in the tub/water?
• What infection control and sanitary practices are used?
• Under what circumstances would you ask me to get out of the water? What’s the emergency plan if something changes quickly?

👀 Read Penny’s full summary of the evidence for more on waterbirths

Is it true that I should avoid dyeing my hair during pregnancy?

Whether or not to avoid hair dye is one of those pregnancy questions that seems to constantly be resurfacing, often accompanied by strong opinions and little explanation. So if you’ve heard “avoid it altogether,” “I got my hair bleached and dyed every 8 weeks throughout pregnancy,” or even “it’s fine if you don’t let too much dye sit on your scalp,” – here’s what the data actually says, so you can decide what’s best for you. 

📚The tl;dr from the evidence: The research that’s been done on hair dye used during pregnancy in standard hair coloring treatments suggests that it’s very unlikely to cause any harms to you or your baby. Human studies don’t demonstrate increased risk of outcomes like miscarriage or birth defects, animal studies looking at very high doses haven’t suggested a risk of birth defects, and clinical organizations including ACOG conclude that hair dye use is unlikely to be toxic for your baby. 

The main reason for concern would be the chemical exposure/absorption, but only a very small amount of the chemicals from the dye are actually absorbed into your body via the scalp. Good ventilation is always a best practice to avoid prolonged exposure to chemical fumes, but for standard coloring treatments in a salon, there hasn’t been evidence to demonstrate reason for concern. 

For women who prefer to take a more conservative approach, waiting until after the first trimester, minimizing scalp contact with treatments like highlights/balayage, and dyeing in a well ventilated area. 

However – there are non-dye related hair treatments, like some keratin/smoothing treatments, where the data demonstrates more need for caution - as some smoothing products can release chemicals like formaldehyde when heated, and some studies have suggested an increased miscarriage risk with formaldehyde exposure. If you’re considering one of these treatments, check the ingredients of the products that will be used and ask your stylist about the salon ventilation. 

👀 Read Penny’s full summary of the evidence for more on hair dye

What’s baby aspirin, how is it different from normal aspirin, and why would my doctor suggest I take it?

Hearing about aspirin during pregnancy often comes as a surprise, especially when women are largely told to avoid medications, and that OTC meds like ibuprofen or aspirin should be avoided if possible. However, “baby aspirin,” as low-dose aspirin is often referred to, has a significant body of evidence supporting its use in some scenarios - so here’s a breakdown of when, where, and why it might be recommended to you. 

📚The tl;dr from the evidence: Low dose aspirin, or “baby aspirin,”  is a lower dose of regular aspirin – commonly 81mg, as compared to 325mg in regular strength aspirin. Low dose aspirin use is not universally recommended during pregnancy, but has demonstrated real benefit in helping to prevent or delay preeclampsia (a very serious pregnancy complication) in women at higher risk for the condition.

For women with risk factors (like a history of preeclampsia, chronic hypertension, carrying twins+, higher age during pregnancy, and other health history/demographic factors), research (including randomized trials!) and clinical guidelines support starting a baby aspirin regimen after 12 weeks but before 16-20 weeks. The research shows strong safety when used as recommended, with low likelihood of serious complications or adverse outcomes for the baby. 

The main concern many women have is a cited concern about maternal bleeding around delivery; some data has shown a slightly elevated risk in increased bleeding or postpartum hemorrhage, so in practice, your doctor is weighing this small bleeding signal against the potential benefits in preventing or delaying preeclampsia. 

So why is “regular” aspirin not recommended in pregnancy, but low dose aspirin can be so beneficial? Dose, timing, and the reason for taking the medication matters. The recommendation to avoid aspirin and other NSAIDs during pregnancy is due to the observed risk of rare but serious fetal kidney problems with use of regular strength aspirin – the risk of taking the medication may be greater than the potential benefit for general pain relief, and preferred alternatives exist; whereas low dose aspirin use (about 25% of the strength of regular aspirin) with existing preeclampsia risk factors under the guidance of a clinician is shown to generally carry higher benefits than risks.  

👀 Read Penny’s full summary of the evidence for more on low dose aspirin

🤓 Zenith's top read of the week

Bonus: what the Zenith team found interesting this week. Think cool pregnancy research or recently published studies, news in pregnancy health and policy, and more!

Postpartum as a Window of Opportunity to Improve Women’s Cardiovascular Health (Elizabeth Jensen & Asma Ahmed, Hypertension) - February is American Heart Month, so it’s timely to see this editorial piece about women’s heart health during the postpartum period – particularly for women who experienced a hypertensive disorder of pregnancy (like high blood pressure or preeclampsia). 

The piece focuses on this study, published late in 2025, about whether blood pressure between 15-90 days postpartum can be associated with hypertension or cardiovascular disease later in life. They had two interesting findings - one was that the postpartum readings were indeed associated with post-pregnancy hypertension. The second (arguably, more structurally concerning) finding was that despite the importance of this measure as a sign of cardiovascular health, ~40% of the women in their sample did not have a blood pressure reading recorded during this period (in contrast to current ACOG guidelines, which recommend multiple postpartum blood pressure readings, including one in the 15-90 day period). 

Heart disease is the leading cause of death for women in the US, so it’s absolutely critical that women receive the right screenings and monitoring during the postpartum period. Catching the signs early can help immensely in making sure women get the care and support they need. 

Key excerpt:

“The risk of incident hypertension after a hypertensive disorder of pregnancy is high in the first year postpartum especially for those with elevated systolic blood pressure postpartum. Despite this, for many participants, blood pressure was not measured within 15–90 days postpartum.”