Can I keep using my period tracker when I'm pregnant?

You can, but it should change what it does. There's no period to predict during pregnancy, so a good tracker switches from forecasting a cycle to showing your gestational week and an estimated due date, and pauses the period countdown. It's a calm companion for following your weeks — not a replacement for prenatal care.

How is a due date calculated?

The classic method (Naegele's rule) counts 280 days — 40 weeks — from the first day of your last menstrual period, assuming a roughly 28-day cycle with ovulation around day 14. Gestational age is then counted from that last period. It's an estimate: only a fraction of people recall their exact last-period date, and ovulation timing varies, so the real date can differ.

Why doesn't my due date match my ultrasound?

Because a first-trimester ultrasound measures the baby directly and is more accurate than dating from your last period. Clinical guidance (ACOG) recommends using the ultrasound date when it differs from the last-period estimate by more than 7 days in the first trimester. That's why a tracker should let you set your due date from a scan rather than locking you to the calendar estimate.

When do periods come back after pregnancy?

It varies widely. If you're not breastfeeding, periods often return within 6–12 weeks; exclusive breastfeeding can delay them for months. The first few cycles are frequently irregular before settling. So postpartum is another stretch where tracking the return calmly beats expecting a precise prediction.

Using a period tracker during pregnancy: what to know

When you’re expecting, there’s no period to predict — but your dates still matter. Here’s how gestational weeks and a due date are estimated, and what a tracker should switch to.

Short answer: during pregnancy a period tracker should stop tracking periods. There’s no cycle to forecast — what matters now is your gestational week and an estimated due date, both counted from your last period. A thoughtful app pauses the countdown, shows your week and due date instead, and lets you correct the date from an ultrasound. It’s a calm companion, never a substitute for your clinician.

How a due date is estimated

The long-standing method is Naegele’s rule: add 280 days — 40 weeks — to the first day of your last menstrual period (LMP). It assumes a roughly 28-day cycle with ovulation near day 14, and gestational age is counted from that LMP. So when an app or midwife says “you’re 9 weeks,” they mean nine completed weeks since your last period — which is a week or two before conception actually happened. That’s the standard convention, not a mistake.

Why it’s an estimate, not a deadline

Two things make the LMP method approximate. First, only about half of people recall the exact first day of their last period. Second — the familiar theme across cycle tracking — ovulation timing varies, so conception may not have happened on the textbook day. (It’s the same reason ovulation is hard to pin to a calendar.) A first-trimester ultrasound measures the baby directly and is more accurate, to within about 5–7 days.

When the ultrasound date wins

Clinical guidance (ACOG) is clear: if a first-trimester ultrasound differs from the last-period estimate by more than 7 days, the ultrasound date is used. That’s exactly why a tracker shouldn’t hard-code the calendar math — it should let you enter the due date your clinician gives you, and recalculate your weeks from it. Dew’s pregnancy mode does this: it estimates from your last period by default, and lets you set a due date from a scan to override it.

The trimesters, briefly

First trimester: through 13 weeks 6 days.
Second trimester: 14 weeks 0 days to 27 weeks 6 days.
Third trimester: 28 weeks until birth — “term” being 37–42 weeks.

What a tracker should do while you’re expecting

The honest design is to switch modes. No fertile window, no “period in N days” — instead, a gentle view of your current week, an estimated due date you can correct, and space to log how you’re feeling. That’s what pregnancy mode in Dew shows: your week, a due-date estimate with a clear “this is an estimate — confirm with your clinician” note, and a quiet pause on cycle predictions until you’re ready to turn them back on. It’s a companion to prenatal care, not a stand-in for it.

And afterwards

Postpartum, periods return on their own timeline — often 6–12 weeks if you’re not breastfeeding, frequently later if you are, and usually irregular at first. So the same principle carries over: track the return calmly rather than expecting an exact prediction out of the gate.

The bottom line

A period tracker can absolutely stay useful through pregnancy — by becoming something else for a while. Expect a due date that’s an estimate (and defer to your ultrasound), think in gestational weeks counted from your last period, and let the app pause the cycle forecasting it can’t honestly do. Then it’s a calm week-by-week companion alongside your real care.

Your pregnancy dates and notes are deeply personal. Dew keeps everything on your device, never uploaded or sold — and when you’re ready, the cycle guide covers what comes next.