When to Start Trying: Age, Health, and Timing

You and your partner are saying "next year" or "soon", and you have noticed that "soon" is starting to need a date. This post is about how to start trying to conceive in a way that takes age, health, and life into account, without turning the decision into a panic about your eggs.

There is no objectively perfect month to start trying. There are better and worse windows, and almost everything that makes a window "better" is something you can prepare for in the three months before it begins. That preparation, not the calendar itself, is the part of timing you actually control.

The honest answer

The better window to start trying is roughly three months after you have addressed the things that change outcomes. Folic acid started, smoking stopped, medications reviewed for teratogenicity, baseline conditions like diabetes and thyroid checked and optimised. The worse window is starting unsupplemented, on a teratogenic medication, with an untreated chronic condition. The distance between those two windows is twelve weeks of preparation, not a different month or a different year.

This is a controllable question, not a fate question. I see two equally common mistakes in clinic, couples in their late thirties waiting "one more year" three years in a row, and couples in their late twenties convinced their fertility is already collapsing. Both groups are using fear to make a decision that needs information.

Age, what the data actually says

Fecundability is the technical term for the chance of pregnancy in any one cycle. It declines gradually from the late twenties, more steeply from the mid-thirties, and sharply from the late thirties.¹ The numbers below are population averages drawn from large historical and modern datasets, and they describe risk rather than certainty.

Approximate live-birth per cycle, in couples without known fertility issues:

• Age 25 to 29: around 25%
• Age 30 to 34: around 20%
• Age 35 to 37: around 15%
• Age 38 to 40: around 10%
• Age 42 and above: much lower, often under 5%

Cumulative natural live-birth by twelve months of trying, in the same population:

• Under 35: roughly 85%
• 35 to 37: roughly 75%
• 38 to 40: roughly 55%²,³

Miscarriage risk follows a similar curve. Around 10 to 15% under age 30, around 20% at 35, around 40% at 40, and above 50% from age 42.⁴

These are not your numbers. They are population numbers, and they tell you about the shape of the curve, not your specific position on it. Cycle regularity, ovulation, weight, smoking status, and known conditions all shift the curve for an individual couple. The number that matters in clinic is rarely the population average. It is your AMH, your antral follicle count, your cycle pattern, and your partner's semen analysis if that becomes relevant.

Age and the "should we freeze eggs" question

This is mostly a different conversation, and a different post. The short version, because it comes up at every consult in this age band, is that if you are 33 to 36 and not in a position to start TTC, a single visit with a fertility doctor for AMH, antral follicle count, and a frank conversation about timelines is reasonable. Egg-freezing success is age-driven in a way most marketing materials do not communicate well. Freezing at 33 has substantially higher yield per cycle than freezing at 38. More eggs are retrieved, and more of them are chromosomally normal. The deeper version of this conversation belongs to the post on ovarian reserve testing in the diagnosis section.

Health, what to address before, not during

Several conditions are easier to optimise before conception than during pregnancy. If any of these apply, the timing of TTC genuinely depends on getting them in range, not just on the calendar.

Diabetes: NICE recommends HbA1c below 48 mmol/mol (6.5%) before conception in people with type 1 or type 2 diabetes, where this is achievable without problematic hypoglycaemia.⁶ Higher HbA1c at conception is linked to congenital malformations and miscarriage in a dose-dependent way. This is one of the very few situations where pushing the TTC start date back by a few months measurably changes outcomes.

Thyroid: Hypothyroidism well controlled, with TSH ideally below 2.5 mIU/L, before conception in people already on levothyroxine. Untreated or undertreated hypothyroidism is associated with miscarriage and impaired neurodevelopment, and the fix is often a blood test and a small dose adjustment.

Blood pressure: ACE inhibitors and ARBs should be switched to pregnancy-safe alternatives such as labetalol, nifedipine, or methyldopa before conception. This is a GP appointment, not an emergency.

Mental health: Most SSRIs can be continued safely, sertraline in particular. Untreated depression in pregnancy has its own real risks. The right time to review is now, not at six weeks pregnant.

Weight: BMI in the 19 to 30 range improves time to pregnancy and reduces gestational diabetes and pre-eclampsia risk. That said, I will say this clearly because it matters: weight loss attempts should not delay TTC indefinitely in someone in their late thirties. The biology does not pause while you optimise.

Routine screening: Cervical screening up to date. Rubella, varicella, pertussis, and seasonal flu immunity confirmed. These are easy to forget and worth checking. The pre-conception checklist post covers the full list in more detail.

The 3-month rule

One full sperm cycle is about 74 days, and one full follicle maturation cycle is about 85 days.⁵,⁷ In both cases, lifestyle changes made today show up in gamete quality about three months later. The sperm being produced this week reflects the last three months of someone's smoking, alcohol, and weight. The follicle that ovulates this cycle was selected roughly three months ago.

Practically, this means aiming to be at least three months into folic acid, supplements, lifestyle changes, and medication review before the first cycle of trying. Three months is short enough to be motivating, and long enough that the biology has actually moved. If you start trying tomorrow without that window, you have not done anything wrong, you have just started before the changes took effect.

Life timing, the real-world considerations

There are practical, non-medical inputs to the "when" question that I find clinicians often skip. They are worth naming.

In the UK, statutory maternity pay requires 26 weeks of continuous employment by the qualifying week. If you are in a new job, that is a real and dateable constraint, not a soft preference. NHS-funded fertility treatment varies significantly by Integrated Care Board, and if you are 35 or older and may need referral, the geography of your registered GP affects what is available. In the US, insurance changes around employer transitions affect what is covered, and many fertility benefits have waiting periods or pre-existing condition clauses.

Housing, partner availability, the support network around you. These are real considerations, not weaknesses. "When is the right time for us" is a different conversation from "is my body ready," and both deserve airtime. The couple-alignment post covers the relationship side of this question more directly.

When to start sooner rather than later

There are situations where delaying the start of TTC is the wrong default. Talk to a GP or fertility doctor early, ideally before you start trying, if any of these apply:

• Age 35 or older without a child already
• A known diagnosis of PCOS, endometriosis, fibroids, or prior pelvic surgery
• Cycles consistently shorter than 21 days or longer than 35 days, or absent
• A partner with a known fertility issue, prior testicular surgery, mumps orchitis after puberty, chemotherapy, or a varicocele
• Recurrent miscarriage history, defined as two or more consecutive losses

In any of these, the earlier conversation is not a panic conversation. It is a triage one, and it shortens the timeline by exactly however long you would otherwise have waited.

A starter framework

If you want a practical timeline, the four steps below are what I usually walk through at the first preconception consult.

1. Today: Start a prenatal containing 400 micrograms of folic acid. Use 5 milligrams per day if BMI is 30 or above, you have diabetes, you take certain anticonvulsants, you have had a previous pregnancy affected by a neural tube defect, or you have sickle cell disease or thalassaemia.
2. This week: Book a GP "planning a pregnancy" appointment. Your partner starts a male preconception multivitamin (zinc, selenium, vitamins C and E, folate, often CoQ10).
3. This month: Stop smoking and vaping. Reduce alcohol. Get any chronic conditions reviewed and adjusted. Confirm vaccinations and cervical screening.
4. Month three: Ready to start trying. The folic acid is on board, the cycle pattern is documented, and the male partner has completed one full sperm-production cycle on the new lifestyle.

This framework is what makes "soon" turn into a date that is actually defensible.

What's normal, what's not, when to call your doctor

Needing a few months to feel emotionally ready is normal. Having mixed feelings, particularly if this is a first pregnancy or comes after a long deliberation, is normal. Not getting pregnant in the first one to three cycles after starting is normal, and is not the start of an infertility story for most couples.

What is common but worth noticing is a change in cycle pattern during the preparation period, particularly after stopping hormonal contraception. Cycles may take a few months to settle into a regular rhythm. The exception is Depo-Provera, where return of ovulation can take six to twelve months.

What is not normal, and is worth seeing a GP about before starting, is any of the red flags listed above. The triage conversation about how to start trying to conceive safely is short, often a single appointment, and it usually shortens rather than complicates the path.

What's next

• If you want the full preconception checklist for both partners: pre-conception checklist, what to do before you start trying
• If you want a realistic sense of how long it usually takes: how long it usually takes to get pregnant
• If you and your partner need to align before setting a date: should we start trying, the couple conversation
• If you are starting older or have a red flag from the list above: when to see a fertility doctor

Sources

1. National Institute for Health and Care Excellence. Fertility problems: assessment and treatment. NICE Clinical Guideline CG156. Published 2013, updated 2017. https://www.nice.org.uk/guidance/cg156
2. van Noord-Zaadstra BM, Looman CW, Alsbach H, Habbema JD, te Velde ER, Karbaat J. Delaying childbearing: effect of age on fecundity and outcome of pregnancy. BMJ 1991;302(6789):1361-1365. https://pubmed.ncbi.nlm.nih.gov/2059713/
3. Dunson DB, Colombo B, Baird DD. Changes with age in the level and duration of fertility in the menstrual cycle. Human Reproduction 2002;17(5):1399-1403. https://academic.oup.com/humrep/article/17/5/1399/685216
4. Magnus MC, Wilcox AJ, Morken NH, Weinberg CR, Håberg SE. Role of maternal age and pregnancy history in risk of miscarriage: prospective register based study. BMJ 2019;364:l869. https://www.bmj.com/content/364/bmj.l869
5. Practice Committee of the American Society for Reproductive Medicine. Optimizing natural fertility: a committee opinion. Fertility and Sterility 2017;107(1):52-58. https://www.asrm.org/practice-guidance/practice-committee-documents/optimizing-natural-fertility-a-committee-opinion-2022/
6. National Institute for Health and Care Excellence. Diabetes in pregnancy: management from preconception to the postnatal period. NICE Guideline NG3. Published 2015, updated 2020. https://www.nice.org.uk/guidance/ng3
7. ESHRE Guideline Group on Female Fertility Assessment. ESHRE guideline: female fertility assessment. Human Reproduction Open 2024. https://www.eshre.eu/Guidelines-and-Legal/Guidelines